For many GMAT test takers, Data Sufficiency questions are the most difficult questions on the GMAT. But what do the hardest GMAT Data Sufficiency questions look like? What skills and concepts do they test? What do they have in common? What Data Sufficiency strategies can we use to get these challenging GMAT Quantitative questions right?

In this article, I’ll go over the five hardest GMAT Data Sufficiency questions, what you’ll need to know to solve them, how to approach them on test day, and what we can learn from hard GMAT Quant questions about mastering Data Sufficiency.

How We Found These GMAT Data Sufficiency Questions

To gather the hardest GMAT questions, our GMAT experts took advantage of the computer adaptive algorithm used on the test. Over the course of the test, the difficulty levels of questions change based on how well you performed on previous questions. Get a few questions right, move up a difficulty level. Get a few questions wrong, move down a difficulty level. By the end of the test, every test taker should be presented with questions that perfectly match their ability.

Our GMAT experts took the practice tests on the GMATPrep software multiple times without missing a single question on the Quantitative section. We collected the questions they received into a master list of the hardest GMAT Quantitative questions. We then looked at activity on various online forums to determine which of these hard GMAT math questions test takers struggled with the most from each question type. This left us with the five hardest GMAT Data Sufficiency questions out there, ready for you to study!

GMAT Data Sufficiency Question 1

This particular problem gives us four different numbers on a number line ($A$, $B$, $C$, and $D$) and tells us the distance between two sets of points ($A$ ↔ $B$ and $C$ ↔ $D$). We should also note that these points are not necessarily in alphabetical order. Whenever we have GMAT Quant questions dealing with shapes, graphs, number lines, etc., it’s a really good call to draw out examples — this is the visual equivalent of plugging in numbers.) Applying this trick here, and remembering that the distance between $A$ and $B$ is longer than between $C$ and $D$, we see that our line could look like this:
this:
this:
and so on.

We need to find the distance between $B$ and $D$. This means that we need to gather information

1. about the order of the points
2. about how our first set of points ($A$ and $B$) relate to our second set of points ($C$ and $D$)

Statement 1

If $A$ and $B$ are two different points and are both the same distance from $C$, this means that the distance between $A$ and $C$ must also be 18 and that $C$ must be directly between the two points like so:
We also know that $D$ is only 8 away from $C$, so it is closer to $C$ than either $A$ or $B$. However, we still don’t know where $D$ is compared to these points. It could be between points $A$ and $C$, making it 26 away from $B$:
or between points $C$ and $B$, making it 10 away from $B$:
Since we don’t know whether the distance between $B$ and $D$ is 26 or 10, Statement 1 is insufficient.

Statement 2

Statement 2 tells us that $A$ is to the left of $D$. Well, $A$ is to the left of $D$ in both of the number lines above, and the distance between $B$ and $D$ is not the same in either. So this doesn’t tell us much. If the statement told us that $A$ was directly to the left of $D$, this might be a little more helpful … but it didn’t and it isn’t. Statement 2 is insufficient.

BOTH

Well, we already established that $A$ is to the left of $D$ (fulfilling Statement 2) in both of the number lines we created to fulfilling Statement 1, so even with the information from both statements, we don’t know whether the distance between $B$ and $D$ is 26 or 10. Since we still can’t solve for a single solution, the correct answer is E: Statements 1 and 2 TOGETHER are NOT sufficient to answer the question.

GMAT Data Sufficiency Question 2

Whenever we have a word problem, like this one, we want to translate the words into math. Scanning over the problem, we see the phrases “36 or fewer” and “more than 36” — these are classic signs that we’re dealing with inequalities. This particular problem gives us two scenarios for calculating how much Bob is paid based on how many total items he produces in a given week (one for 36 or fewer items, one for more than 36 items), so we want to create two equations: one for each scenario. Letting $i$ = the number of items Bob makes in a given week, we can translate our first scenario as

$\If i ≤ 36\, \then \total \pay=x×i$

Our second sentence is a little more complicated. If Bob produces more than Bob is paid $x$ for the first 36 items (or $36x$). Then for all of the items after 36 (or $i-36$), he is paid $1.5x$ (or $1.5x×(i-36)$). Putting that together,

$\If i > 36\, \then \total \pay=x×36 + 1.5x×(i-36)$

So we have two equations, each with three variables ($i$, $x$, and $\total \pay$) … which means we need a bunch of information to figure out an answer. To figure out a value for $i$, we need information about

• which of the two equations to use
• the value of $x$
• the total pay

Statement 1

This statement tells us how much Bob was paid last week, but it doesn’t tell us anything about the specific value of $x$ or which of the two equations we should use. So we could have:

$i=1 \and x=480 → 480=480×1$

or

$i=32 \and x=15 → 480=15×32$

or

$i=76\ \and x=5 → 480=5×36 + 1.5(5)×(40)$

and so on. Statement 1 is insufficient.

Statement 2

This one tells us how much Bob was paid this week, and it compares the number of items he produced this week to the number he produced last week. Well, we don’t know anything about how many items Bob produced last week, so the last piece of information doesn’t tell us much about $x$ — he could have produced 1 item last week and 3 this week or 100 items last week and 102 this week. And, like in Statement 1, we don’t know whether or not $i$ is greater than 36, so we don’t know which statement to use. So we could have:

$i=4 \and x=145 → 580=145×4$

or

$i=29 \and x=20 → 580=20×29$

or

$i=41\, \and x=13{1/3} → 580=13{1/3}×36 + 1.5(13{1/3})×(5)$

and so on. Statement 2 is insufficient.

BOTH

What if we put the two statements together? Well, now we know something: the additional two items Bob produced this week earned him \$30 more than he earned last week. This means that Bob earned an extra /$15 per item. But we’re still missing a key piece of information: which scenario are we dealing with?

1. Did Bob produce 36 or fewer items this week? If so, then both items were produced at a rate of $x$, so that $x=15$.
2. Did Bob produce at least 38 items this week? If so, then both items were produced at a rate of $1.5x$, so that $1.5x=15$ → $x=10$?
3. OR did Bob produce exactly 35 items last week and 37 items this week? If so, then the first item was produced at a rate of $x$ and the second item was produced at a rate of $1.5x$, so that $x+1.5x=30$ → $2.5x=30$ → $x=12$.

We’ve got a few options here, so let’s try each individually. Remember, we want to solve for the number of items Bob produced last week, so we’ll use that equation:

1. $x=15$, $480=15i$ → $i=32$
2. $x=10$, $480=36(10)+1.5(10)(36-i)$ → $480=360+15(36-i)$ → $120=15(i-36)$ → $8=i-36$ → $i=44$

We already have two possible solutions, so we don’t need to look at our third, more complicated option. We cannot determine whether Bob made 32 or 44 items last week, so we cannot solve the problem with both statements. The correct answer is E: Statements 1 and 2 TOGETHER are NOT sufficient to answer the question.

GMAT Data Sufficiency Question 3

Reading through the question, we see that we’re dealing with a group of houses where some have a swimming pool and some have a patio. Scanning over the statements, we see that some houses have only a pool, some houses have only a patio, some have neither, and some have both. Almost anytime we see the word “both” in GMAT Quant questions, we’re dealing with an overlapping sets problem — we are looking at two criteria (here, having a pool and having a patio) and where they overlap (here, having “both” a pool and a patio).

Overlapping sets problems have a lot of information, so it’s really easy to get lost in them. A good trick is to use a visual representation to keep track of what you know:

• For two overlapping criteria, use a table, where each axis represents one criterion.
• For three overlapping criteria, use a venn diagram, where each circle represents a criterion.

Here, we have two overlapping sets, so we’re going to use a table. We’ll go ahead and fill in only what was stated directly in the question. We want to find the total number of houses that have a Pool, so we’ll represent that in our table as $x$:

Because of the way we’ve set the table up, the two numbers in each row should add up to the total at the end of the row and the two numbers in each columns should add up to the total at the bottom of the column. This means that if we have at least two of the three values in each row or column, we should be able to solve for the third. Looking at our table, we see that our total row along the bottom has two values. If there are 75 houses in total and 48 of those houses have patios, 75 – 48 = 27 of those houses must not have patios. We can go ahead and fill that information in our table:

Doesn’t seem like we can get much more out of our table at this point, so we’ll move on to our Statements.

Statement 1

To start we’ll fill in the information directly given in the statement:

We see that our first column has two values, so we should be able to solve for the third. If there are 58 houses with patios and 38 of those houses do not have pools, 48 – 38 = 10 of those houses must have pools:

Looking at the row and the column that contain $x$, we see that we only have one number value for each, meaning that we can’t solve for $x$. Statement 1 is insufficient.

Statement 2

This statement doesn’t give us any concrete numbers to work with, but it does tell us that two of our values (houses with both pools and patios and houses with neither pools nor patios) are equal to each other. When we know that the same number shows up in two places, but we don’t know what that number is, it’s a good idea to represent that number with a variable — if we represent both values as, say, $n$, we know that they are the same number and can combine or eliminate them down the line:

Now we’re getting somewhere! We don’t have two number values in any row or column, but we can use both the top row and the second column to represent No Patio/Pool with variables: if there are $x$ total houses with pools and $n$ of those houses have patios, $x-n$ must not have patios, and if there are 27 total houses that do not have patios, and $n$ of those houses do not have pools, $27-n$ must have pools:

Since the number of houses with no patio and a pool equals both $x-n$ and $27-n$, we can set the two equal to each other to solve for $x$:

$x-n=27-n$

$x=27$

We were able to determine that 27 houses have pools, which means that Statement 2 is sufficient. The correct answer is B: Statement 2 alone is sufficient to answer the question.

GMAT Data Sufficiency Question 4

Right away, the word “ratio” tips us off that we’re dealing with ratios in this problem, and the word “greater” indicates that we’re dealing with inequalities. However, as we read through the rest of the problem, things start to get a little more confusing: one company, two divisions, full-time and part-time employees … this is a lot to process.

We do see the words “either” and “both” though, which should get some overlapping sets wheels turning in our minds. We see that, like the problem above, we have two criteria: employees can belong to Division X or Division Y and can be full-time or part-time. Since this problem doesn’t have any concrete numbers, it isn’t strictly necessary to make a table like we did in the problem above. However, it can still be helpful to define the relationships between our sets and build equations:

We know that the two numbers in each row should add up to the total at the end of the row and the two numbers in each columns should add up to the total at the bottom of the column. So we can now build 6 different equations:

1. $\Full\-\Time \@ \X + \Part\-\Time \@ \X = \Employees \@ \X$
2. $\Full\-\Time \@ \Y + \Part\-\Time \@ \Y = \Employees \@ \Y$
3. $\Full\-\Time \@ \Z + \Part\-\Time \@ \Z = \Employees \@ \Z$
4. $\Full\-\Time \@ \X + \Full\-\Time \@ \Y = \Full\-\Time \@ \Z$
5. $\Part\-\Time \@ \X + \Part\-\Time \@ \Y = \Part\-\Time \@ \Z$
6. $\Employees \@ \X + \Employees \@ \Y = \Employees \@ \Z$

Now that we have this set up, let’s figure out what the question is asking for. Like with all word problems, we want to translate words into math. Whenever we’re dealing with ratios, we should remember that ratios can (and should) be expressed as fractions:

Is ${\full\-\time \@ \X}/{\part\-\time \@ \X} > {\full\-\time \@ \Z}/{\part\-\time \@ \Z}$?

or in other words, are there more full-time employees for every part-time employee at Division X than at the entire company?

Statement 1

This Statement gives us information about the ratio of full-time employees to part-time employees at Division Y compared to Company Z:

${\full\-\time \@ \Y}/{\part\-\time \@ \Y} < {\full\-\time \@ \Z}/{\part\-\time \@ \Z}$

Now, before we rule this statement out because it doesn’t tell us anything about Company X, let’s see how we can use our equations to substitute X back into the inequality. Looking at equations 4 and 5, we see that we can rearrange the equations to give:

4. $\Full\-\Time \@ \Y = \Full\-\Time \@ \Z – \Full\-\Time \@ \X$
5. $\Part\-\Time \@ \Y = \Part\-\Time \@ \Z – \Part\-\Time \@ \X$

Subbing those into our inequality gives us:

${\full\-\time \@ \Z – \full\-\time \@ \X}/{\part\-\time \@ \Z – \part\-\time \@ \X} < {\full\-\time \@ \Z}/{\part\-\time \@ \Z}$

Let’s think about what we know about fractions. To make a fraction smaller, we need to either

1. decrease the numerator relative to the denominator
2. increase the denominator relative to the numerator

We know that we are decreasing both the numerator and denominator, so we must be decreasing the numerator by a greater percentage than we are decreasing the denominator. This means that the number of full-time employees at Division X is larger relative to the number of part-time employees at Division X than the number of full-time employees at Company Z to the number of part-time employees at Company Z. In other words, the ratio of the number of full-time employees to the number of part-time employees is greater for Division X than for Company Z. Statement 1 is sufficient.

Statement 2

Like with Statement 1, let’s translate this into math:

$\full\-\time \@ \X > {1/2}\full\-\time \@ \Z$

$\part\-\time \@ \Y > {1/2}\part\-\time \@ \Z$

Given equation 5, the second half of our statement also tells us that

$\part\-\time \@ \X < {1/2}\part\-\time \@ \Z$

This means we can write the ratio of full-time employees at Division X as

${>{1/2}\full\-\time \@ \Z}/{<{1/2}\part\-\time \@ \Z}$

or, cancelling the {1/2} in both the numerator and denominator,

${>\full\-\time \@ \Z}/{<\part\-\time \@ \Z}$

To make a fraction larger, we need to either:

1. increase the numerator relative to the denominator
2. decrease the denominator relative to the numerator

Here, we’re doing both: full-time employees at Division X is greater than full-time employees at Company Z and part-time employees at Division X is less than part-time employees at Company Z. This means that

${\full\-\time \@ \X}/{\part\-\time \@ \X} > {\full\-\time \@ \Z}/{\part\-\time \@ \Z}$

which is exactly what we’re trying to solve for. Statement 2 is sufficient.

Since both statements are sufficient to solve the problem individually, the correct answer is D.

GMAT Data Sufficiency Question 5

The word “remainder” tells us that we’re dealing with, what else, a remainder problem. Remainder problems scare a lot of students because they don’t involve an easy to use/memorize formula. However, this means that we have a great opportunity to plug in numbers.

Even though this isn’t technically a “word problem”, we still need to translate the words into math to build an equation:

${(n-1)(n+1)}/24 = \? \| \R\: r$

Let’s make a note that $n$ must be a positive integer and move on to our statements.

Statement I

This statement tells us that $n$ is not divisible by two — in other words, it’s telling us that $n$ is odd. Let’s try plugging in numbers. When we select numbers to plug in, our goal is to prove that the statement is insufficient: in other words, we want to pick numbers that will give us different results. We also want to pick numbers that are easy to work with to save time.

We see that one of the values in our numerator is (n-1), which means that picking 1 will give us a zero in our numerator. That seems like it’ll give us an interesting result, so we’ll give it a shot:

${(1-1)(1+1)}/24$

${(0)(2)}/24$

$0/24$

$0 | \R\: 0$

So when $n=1$, $r=0$. Let’s try our next odd number up, $3$ — based on the size of the denominator, it seems like our numerator will be smaller than the denominator, giving a solution of 0 with positive remainder:

${(3-1)(3+1)}/24$

${(2)(4)}/24$

$8/24$

$0 | \R\: 8$

So when $n=3$, $r=8$. This means that $r$ can be either 0 or 8 given Statement 1. Since we can’t find a single value for $r$, Statement 1 is insufficient.

Statement II

This statement tells us that $n$ is not divisible by three. That knocks $n=3$ out of the running. $n=1$ still works, however, so we know that $r=0$ is still a possibility given Statement 2.

Since we tried only odd numbers last time, let’s try an even number this time to see if that changes things up: we’ll do 2 to keep our numbers easy to work with:

${(2-1)(2+1)}/24$

${(1)(3)}/24$

$3/24$

$0 | \R\: 3$

So when $n=2$, $r=3$. This means that $r$ can be either 0 or 3 given Statement 2. Like before, since we can’t find a single value for $r$, Statement 2 is insufficient.

BOTH

Putting these two statements together, we know that $n$ must be odd and cannot be divisible by 3: so we have 1, 5, 7, 11, etc. These numbers are going to get pretty big pretty fast, so let’s try them from smallest to greatest. We already know that $r=0$ when $n=1$, so we want to find a positive value for $r$ to prove that both statements are insufficient:

${(5-1)(5+1)}/24$

${(4)(6)}/24$

$24/24$

$1 | \R\: 0$

So when $n=5$, $r=0$. That’s the same as when $n=1$. Let’s try the next number up, 7:

${(7-1)(7+1)}/24$

${(6)(8)}/24$

$48/24$

$2 | \R\: 0$

So when $n=7$, $r=0$. We’re starting to see the hints of a pattern here. Let’s try one more, 11, to be sure:

${(11-1)(11+1)}/24$

${(10)(12)}/24$

$120/24$

$5 | \R\: 0$

So when $n=7$, $r=0$. Once we’ve tried at least 4 numbers in a series and confirmed that we’ve done a reasonable job picking numbers that would give us different results, we can usually determine that we have a pattern. Here, we can say confidently that given Statement 1 and Statement 2, $r$ will always be 0. This means that the correct answer is C: BOTH statements together are sufficient.

Key Takeaways: Learning From The Hardest Data Sufficiency Questions

So what can the hardest GMAT Quantitative questions teach us about GMAT Data Sufficiency questions in general?

1. Visuals — drawings, tables, Venn diagrams, graphs, what have you — are our friends, and not only on Geometry questions. On the GMAT, advanced quant questions are hard to conceptualize, and drawing things out keeps us from having to keep track of a lot of complicated relationships in our heads.
2. Whenever we have words, we need to translate them into math. Like visuals, building equations helps us take hard GMAT math questions and distill them into something we can work with. Use math-y keywords, like “greater than”, “equal to”, “divided by”, etc. to break sentences down into their component parts.
3. The hardest GMAT Data Sufficiency questions often involve more logic than simple math, especially around number sense concepts. Being comfortable making inferences based on what we know can save us a lot of time compared to slogging through a bunch of proofs.
4. That said, picking numbers to plug in is a great Data Sufficiency strategy that can help us avoid overthinking a problem or confirm our logic. Always pick numbers that you think will yield two different solutions, making the statement insufficient.

What’s Next?

What are the math concepts tested on the GMAT? The best GMAT math tricks and shortcuts? The most important Data Sufficiency tips? These articles expand on the concepts used in these five problems, explaining what you need to know about GMAT Data Sufficiency before test day.

Looking to improve your Quant score? This article explains what exactly a good GMAT Quantitative score is.

If you’d like similar analyses of the hardest questions from other GMAT question types, check out our post on the five hardest Sentence Correction questions.

The post The 5 Hardest GMAT Data Sufficiency Questions appeared first on Online GMAT Prep Blog by PrepScholar.

For many GMAT test takers, hard Sentence Correction questions are what stand between them and a top Verbal score. But what do the hardest GMAT Sentence Correction questions look like? What skills and concepts do they test? What do they have in common? What strategies can we use to get these GMAT Verbal questions right?

In this article, I’ll go over the five hardest GMAT Sentence Correction questions, what you’ll need to know to solve them, how to approach them on test day, and what we can learn from hard GMAT questions about mastering Sentence Correction.

How We Found These GMAT Sentence Correction Questions

To gather these questions, our GMAT experts took advantage of the computer adaptive algorithm used on the test. Over the course of the test, the difficulty levels of questions change based on how well you performed on previous questions. Get a few questions right, move up a difficulty level. Get a few questions wrong, move down a difficulty level. By the end of the test, every test taker should be presented with questions that perfectly match their ability.

Our GMAT experts took the practice tests on the GMATPrep software multiple times without missing a single question on the Verbal section. We collected the questions they received into a master list of the hardest GMAT Verbal questions. We then looked at activity on various online forums to determine which of these questions test takers struggled with the most from each question type. This left us with the five hardest GMAT Sentence Correction questions out there, ready for you to study!

GMAT Sentence Correction Question 1

When we have a sentence as long and confusingly ordered as this one, a really good move is to eliminate anything unnecessary to the grammatical structure of the sentence. This can help us to figure out what exactly the sentence is actually saying and pinpoint errors in sentence construction. Some good candidates for elimination are parts of the sentence set off by commas and long descriptive phrases — we should be able to remove anything that provides “extra” information and still have a grammatical sentence. For example,

This Thursday afternoon, I went to the potluck at Julianne and Michelle’s apartment.

can be simplified to

This Thursday afternoon, I went to the potluck at Julianne and Michelle’s apartment.

without messing up the grammar.

Let’s do the same thing with our sentence:

Based on records from ancient Athens, each year young Athenian women collaborated to weave a new woolen robe that they used to dress a statue of the goddess Athena and that this robe depicted scenes of a battle between Zeus, Athena’s father, and giants.

or, simplified,

Young Athenian women collaborated to weave a new woolen robe and that this robe depicted scenes of a battle.

Much simpler! Now we see that the sentence is divided into a series of two parts (“Young … woolen robe” and “that … battle”) separated by a conjunction (“and”). We also see that the last portion of the sentence is not underlined. Whenever we see that we have a series where part of the series is underlined and part is not, we should immediately start looking for parallelism issues.

Looking at our two parts, we see one big difference in grammatical structure:

• Young Athenian women collaborated to weave a new woolen robe
• that this robe depicted scenes of a battle

Since the part of our series that isn’t underlined begins with “that”, we need the other part to include “that” to match. Looking through the answer options, only D uses the word “that” twice, giving us correct parallel structure.

GMAT Sentence Correction Question 2

This question is tricky because it relies heavily on having an idiom memorized rather than applying grammatical concepts — if you don’t know the idiom, it’s going to be much harder to get the question right. The GMAT has a few classic idioms that are bound to show up on any test. “Consider X, Y” is one of them. “Consider as” and “consider to be” are both used colloquially, but neither is grammatically correct. This allows us to eliminate C, D, and E right off the bat, leaving us with A and B.

Like the previous question, we’re dealing with a pretty complex sentence. Right away, we see that “With surface … Fahrenheit” is a descriptive phrase set off by a comma, so it shouldn’t be necessary to the grammar of the sentence. Let’s go ahead and eliminate it from our sentence to simplify things:

Jupiter’s moon Europa has long been considered far too cold to support life, and with 60 square miles of water thought to be frozen from top to bottom.

Looking at A, we see that the sentence is divided into two clauses (“Jupiter’s … life” and “with … bottom”) separated by a conjunction (“and”). However, this time we have a comma before our conjunction. Whenever we have two clauses connected by a comma + a conjunction, both clauses need to be independent — they need to be able to stand on their own as sentences. For example,

Rosalind put on boots and wrapped a scarf around her neck.

is fine, but

Rosalind put on boots, and wrapped a scarf around her neck.

is not — “wrapped a scarf around her neck” doesn’t work as it’s own sentence. We would need to add a subject:

Rosalind put on boots, and she wrapped a scarf around her neck.

Let’s see if both parts of our sentence can stand alone as independent clauses:

• Europa has long been considered far too cold to support life.
• With 60 square miles of water thought to be frozen from top to bottom.

We see right away that the second part of our sentence is not a complete thought. We can eliminate A, leaving us with B as the correct answer.

Now, if we didn’t recognize the idiom error in the sentence, we can eliminate A based on the reasoning above and continue to use sentence structure to eliminate answers. We see that a few of our answer choices have additional pieces of the sentence set off by commas that we can remove:

2. Jupiter’s moon Europa has long been considered far too cold to support life, its 60 square miles of water thought to be frozen from top to bottom.

4. Jupiter’s moon Europa, long considered as far too cold to support life, and its 60 square miles of water thought to be frozen from top to bottom.
5. Jupiter’s moon Europa, long considered to be far too cold to support life, and to have 60 square miles of water thought to be frozen from top to bottom.

As expected, B looks fine. However, we see that D and E don’t make much sense once we remove the “unnecessary” parts of the sentence. We can eliminate both, leaving us with B, C, and a 50% chance of guessing correctly.

GMAT Sentence Correction Question 3

Once again, another long, complicated, heavily punctuated sentence. Sensing a trend? Before we start breaking this sentence apart, however, let’s scan to see if there is anything a little easier to work with in this sentence by looking at the differences between the answer choices. We notice quite a few changes in tense (“providing” vs. “provided”, “milking” vs. “milked”, and “are producing” vs. “produces” “will produce”), but we should also notice something simple: the change from “cows” to “cow”. Whenever we see changes between singular and plural in answer choices, we should immediately look for issues with pronoun agreement. We find our pronoun in the non-underlined portion of our sentence: “For the farmer who takes care to keep them cool”.

Because this part of the sentence isn’t underlined, we know that the sentence must discuss “Holstein cows”, not “the Holstein cow”. We can eliminate B and D.

Now we can look at the structure of the sentence. We see that the sentence begins with a list of three things the farmer does to cause the Holstein cow to produce more milk. A list is just a series of three or more items, which means we need to start thinking about parallelism. Since the first list item is not underlined, we want to match the structure of the second and third items to the first.

Looking at A, we see that the farmer

• takes care to keep them cool
• providing them with high-energy feed
• milking them regularly

So all of our list items involve verbs, but the -ing verbs in the second two list items don’t match any tenses in the first list item. Eliminate A.

Note: some might think that “providing … regularly” might act as a modifying phrase for “takes care to keep them cool”, making an -ing verb a good pick (as in “I quickly called the office, holding my breath and hoping they wouldn’t be closed”). However, it doesn’t make much sense that feeding and milking cows would describe keeping them cool. Similarly, the comma between “feed” and “milking” would not be grammatical outside of a list.

C sets up a similarly confusing list. The farmer keeps the cows

• cool
• provided with high-energy feed
• milking them regularly

“Cool” and “provided” both act as adjectives to describe the cows, but now “milking” doesn’t fit the structure. We can eliminate C, leaving us with E, in which the farmer keeps the cows

• cool
• provided with high-energy feed
• milked regularly

GMAT Sentence Correction Question 4

Sentences that are fully underlined can be intimidating — we aren’t sure of anything in the sentence. However, this is often a blessing in disguise. Since there may be multiple errors in the sentence, there may be multiple opportunities for us to rule out answer options.

Scanning over our sentence, we notice that the sentence sets up a comparison between the existence of legal size limits for cod and haddock and the lack of legal size limits for monkfish. Comparison questions are a favorite on the GMAT since they test a couple things — parallelism and “like/unlike” vs. “as”. Like with a list or a series, things being compared must have parallel structure. This concept is pretty easy to test for on a question-by-question basis, but “like” vs. “as” is another one of the GMAT Sentence Correction rules on idiom and diction we need to have memorized: “like” is used to compare two objects (nouns), while “as” is used to compare two actions (verbs).

Looking at A we see our comparison is between

• There are no legal limits … on the size of monkfish that can be caught

and

• there are [legal limits] for cod and haddock

… which actually works fine — the two things being compared are parallel and are both verbs, which fits with the use of “as”. We can keep A and check our other answers.

B sets up a comparison using “unlike”, so we know we should be comparing two nouns. However, the first part of our sentence doesn’t change at all, giving us a comparison between

• There are no legal limits … on the size of monkfish that can be caught

and

• cod and haddock

So we have a parallelism issue, (comparing a lack of legal limits to a fish doesn’t make much sense), AND we have an issue with idiom and diction (“unlike” can’t compare a verb to a noun). We can eliminate B based on either.

Looking at D and E, our other “unlike” answer choices, we see similar issues. In D, we compare “there are no legal size limits on catching monkfish” with “cod and haddock“. In E, we compare “there are no legal size limits on catching monkfish” with “catching cod and haddock (where “catching” is a gerund, a type of noun). We can eliminate both.

This leaves us with A and C. C doesn’t use “like” or “as”, so we’re out of luck there. It does, however, use the word “which”, which (… get it) should ring some more idiom and diction bells. On the GMAT, the word “which” must refer to the closest noun. For example:

We ran to the store, which made us tired.

is something we might say colloquially — the act of running made us tired. However, on the GMAT, this sentence would indicate that the store itself made us tired.

Looking at our sentence, we see that “which contributes to its depletion by being overfished” refers to monkfish … so monkfish are contributing to their own depletion. That doesn’t make much sense. We can eliminate C, leaving us with A as the correct answer.

GMAT Sentence Correction Question 5

Unlike the monkfish question, this sentence is short, and the underlined portion is even shorter. This means we may not be working with much.

Reading the initial sentence, we should notice the phrase “more fuel-efficient … than”. Once again, we’re dealing with a comparison: the fuel efficiency of small cars in the past vs. the fuel efficiency of small cars in the present.

However, we might notice something sounds odd about the comparison in first sentence. Small cars are “more fuel efficient now than at any time in their production history“. Well … doesn’t “any time in their production history” include “now”? Meaning that small cars are more fuel efficient now than they are … also now?

The impossible comparison is yet another classic GMAT trap, though this one doesn’t show up quite as often as some of the others in these questions. When we compare one thing in a group to the rest of the things in the group, we need to make sure that we exclude the one thing from the group we compare it to, typically using the word “other”. For example:

Keisha scored higher on the the test than all of the students in her class.

is illogical. Keisha is one of the students in her class; this sentence tells us she scored higher than all of them — including herself. We can correct this by excluding Keisha from the class in our comparison:

Keisha scored higher on the the test than all of the other students in her class.

Looking back at our sentence, we can eliminate any answers that use the phrase “at any time”: A, B, and E.

Let’s look at the comparisons made in C and D.

C tells us that manufacturers make “small cars that are more fuel efficient than those [small cars] at any other time in production history”. So the small cars manufacturers make now are more efficient than the small cars they made before.

D tells us that manufacturers make “more fuel efficient small cars than those [manufacturers] at any other time in their production history”. This has a slightly different meaning. Now, the sentence tells us that the manufacturers make more fuel-efficient small cars, not that the small cars they make are more fuel-efficient. So with C, the manufacturers are increasing fuel efficiency, while in D, the manufacturers are increasing the number of cars.

When dealing with issues of meaning on Sentence Correction questions, we want the correct answer to capture the meaning intended by the original sentence. Ignoring the errors in the original sentence, we see that it conveys the idea of “more fuel-efficiency” not “more cars”, making D wrong and C correct.

Key Takeaways: Learning From The Hardest Sentence Correction Questions

So what can the hardest GMAT Verbal questions teach us about GMAT Sentence Correction questions in general?

1. It’s crucial to know the tricky GMAT Sentence Correction rules about idioms, diction, and sentence structure — the most difficult questions tend to target constructions that sound okay in casual conversation but don’t fly on the GMAT.
2. The hardest Sentence Correction questions often have complex sentence structures. Simplifying sentences by eliminating unnecessary elements (descriptive clauses and phrases, for example) can make them easier to decipher.
3. Often dealing with several types of errors within a sentence gives us multiple opportunities to eliminate the wrong answer options. This makes long underlined portions a blessing in disguise.
4. That said, challenging questions often come down to issues of meaning. Knowing GMAT Sentence Correction rules is usually enough to get you down to two answer choices, but on the hardest GMAT questions, you’ll usually need to understand what the sentence is saying to definitively answer the question.

What’s Next?

What are the most important grammar rules for the GMAT? The most common GMAT idioms? The top tips for Sentence Correction? These articles expand on the concepts used in these five problems, explaining what you need to memorize for Sentence Correction before test day.

Aiming for an 800 on the GMAT? This article shares key strategies for getting a perfect score.

If you’d like similar analyses of the hardest questions from other GMAT question types, check out our five hardest Data Sufficiency questions (coming soon).

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