Here’s the Trick to Answering GMAT Data Sufficiency Questions

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The Quantitative Reasoning section of the GMAT includes two question types. The first, called problem-solving questions, should be familiar to you if you’ve ever taken a standardized math test. These multiple-choice questions ask you to solve an equation or a word problem, read a table or graph, or calculate the answer to a geometric problem. You’ve seen questions like this since K-12 school.

And then there’s the second type, which almost certainly will not look familiar to you. They’re called data sufficiency questions, and they ask you not to solve a problem but rather to figure out whether you have enough information to solve a problem. It’s a weird and confusing question, especially when posed under strenuous, high-stakes test conditions.

The GMAT is administered in a computer-adaptive format only, with all quantitative reasoning questions asked in a single section. That means you can see either of these question types at any time, in any order, when you take the exam. You need to be prepared.

What GMAT data sufficiency questions look like

A data sufficiency question offers two statements followed by five answer choices.

Here’s an example of an easy data sufficiency question: What is the value of x?

(1) 2 + x = 4
(2) 10 – x = 8

• A – Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient
• B – Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient
• C – BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient
• D – EACH statement ALONE is sufficient
• E – Statements (1) and (2) TOGETHER are NOT sufficient

To answer, look first at statement 1. Does it provide enough information to answer the question “What is the value of x?” Yes, it does; if you subtract 2 from both sides of the equation, you get x = 2.

What about statement 2? If you subtract 8 from both sides of the equation and then add x to both sides of the equation, you get 2 = x. So yes, statement 2 is also sufficient.

Which answer choice means “statements 1 and 2 are each sufficient on their own?” Answer choice D, so the correct answer to this question is D.

Making sense of the answer choices

OK, that was a pretty easy question, so the format didn’t trip us up too much. But these questions get significantly more difficult. As they do, the unfamiliarity and just plain weirdness of the format can cause some unforced errors.

How to use the AD-BCE method

• First and foremost, resist the temptation to read both statements before you start your work. You’ll be taking in too much information too quickly if you do that, a surefire way to get confused and pick the wrong answer.
• Instead, look only at statement 1. As we did in the first sample question, make a determination whether it’s sufficient before you look at statement 2.
• If statement 1 is sufficient to answer the question on its own, the only possible answer choices are A and D. Those are the only two answer choices that say statement 1 is sufficient on its own.
• If statement 1 is insufficient to answer the question on its own, the only possible answer choices are B, C, and E. So simply by making a determination about statement 1, you’ve improved your odds of guessing the correct answer to one in two or one in three. And you haven’t even looked at statement 2 yet!
• Now look at statement 2 alone — forget all about statement 1. Is statement 2 sufficient? Then the answer is B. If it isn’t, then the answer is either C or E. Now, and only now, do you look at the two statements together. If together they are sufficient, the answer is C. If they aren’t, the answer is E.

And that’s the way to work through each and every data sufficiency question. By using this systematic approach, you will avoid the confusion caused by too much information at once, and you’ll avoid the all-too-common error of mixing up the meanings of the answer choices. Sadly, people get data sufficiency questions wrong all the time simply because they invert the meaning of answer choices C and D. Did we mention that this is a weird and confusing question format?

Here’s a mnemonic for the answer choice groupings: AD stands for “anno domini,” which is Latin for “the year of our Lord.” BCE stands for “before the Christian era,” an alternative to the designation BC (“before Christ”). So together the data sufficiency answer choice groupings cover all of Earth history: AD and BCE!

Sample question

Now it’s time for you to try a question. Remember, these can get tricky.

Jada sold her house through a realtor. What was the sale price of Jada’s house?

(1) The realtor who sold the house earned a 3 percent commission of $10,500 on the sale.
(2) The sum of the sale price and the realtor’s commission was $360,500.

• A – Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient
• B – Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient
• C – BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient
• D – EACH statement ALONE is sufficient
• E – Statements (1) and (2) TOGETHER are NOT sufficient

Using the AD-BCE method, we look at statement 1 first.

• Is it sufficient? Statement 1 tells us that $10,500 is 3 percent of the sale price. We can write a solvable equation to determine the sale price based on that information. Note that we don’t have to; we only need to know that we could. But, indeed, we could; that equation is .03x = 10,500. Divide both sides by .03 to get x = 350,000. The house sold for $350,000. The answer must be A or D.
• Now look at statement 2. Is it sufficient? If we calculated the answer in statement 1, we know that this statement is true; the sum of the sale price ($350,000) and the commission ($10,500) is, indeed, $360,500. But we’re not supposed to figure out whether this statement is true; we’re supposed to figure out whether it provides sufficient information to answer the question.
• We do not have sufficient information to answer the question, Because statement 2 does not provide a commission rate. How the $360,500 is split between sale price and commission depends on the commission rate, which statement 2 does not provide.
• The correct answer to this question, then, is A. Statement 1 is sufficient on its own, statement 2 is not.

Where are the traps?

1. We’ve already suggested one trap: that you will carry information from statement 1 over to your assessment of statement 2, incorrectly assuming that you know the realtor’s commission rate. That’s why you have to forget all about statement 1 when you assess the sufficiency of statement 2 on its own.
2. The second trap is for those who read both statements at the same time, without using AD-BCE. They see a commission of $10,500 in statement 1 and the sum of the commission and sale price in statement 2. They think “Great! I can subtract the commission from the sum of the commission and sale price to get the sale price: $360,500 – $10,500 = $350,000. The answer is C!” The problem with this answer is that C means that neither statement is sufficient on its own, which is untrue; as we explained above, statement 1 is sufficient all by itself. That’s why the correct answer here is A.

What about when you can’t make a decision about statement 1?

So, what happens when you look at statement 1 and realize “I have no idea whether this statement is sufficient to answer the question?” Without being able to make that determination, you can’t use AD-BCE.

The answer: you tweak your approach slightly. Instead of assessing statement 1’s sufficiency, assess statement 2. If it’s sufficient, the correct answer must be B or D. If it’s insufficient, the correct answer must be A, C, or E.

And yes, we have a mnemonic for these answer choice groupings too: in the comic strip Doonesbury, there’s a character named BD who was an ACE quarterback in college. Yeah, it’s not as good as the AD-BCE mnemonic because, well, not everyone reads Doonesbury, but it’s the best we could do. If you come up with a better one, please let us know. Our contact information is below.

You’ll need to practice to prepare for all GMAT questions, but you need to focus particularly on data sufficiency questions because, as we’ve noted repeatedly, the question format is so weird and confusing. The Official Guide to the GMAT contains lots and lots of practice questions — and entire practice tests — that actually appeared on real GMATs, which is what makes it the best (some would argue only) valuable test prep resource available. Get yourself a copy and start practicing a little every day; you should give yourself a minimum of four weeks to prepare for the test.

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