GMAT Tip of the Week

Enough is Enough

(This is one of a series of GMAT tips that we offer on our blog.)

A format unique to the GMAT, Data Sufficiency questions comprise nearly half of the GMAT’s quantitative section and tend to give examines fits simply because of the unique phrasing. Accordingly, a sound understanding of the question format will provide you with a distinct competitive advantage, and may even allow you to enjoy the challenge. True to their name, Data Sufficiency questions ask when the given information is sufficient to answer the overall question. To effectively answer these questions, it is first important to understand exactly what they are asking of you so that you’ll know precisely when you have enough information to provide a satisfactory answer.

Perhaps the easiest way to fully internalize the scope of these questions is to replace the word “is” with the word “must” – the questions are not asking you whether an answer is possible, but rather whether it must be the case. Accordingly, the question “Is x > 5?” isn’t asking “could x be above 5?”, but rather “must x be > 5?”. In this case, it isn’t sufficient to say simply that x could be 8; you’ll need proof that x can only be represented by values above 5. A common trap inherent in the exam involves an application of the following:

Is x > 5?

(1) x² = 64
(2) x > 0

The first statement does indicate that x could equal 8, which would provide the answer “yes”; but that same statement allows for a situation in which x equals -8, which would provide the answer “no”. Because you can use the information to elicit two different answers to the question, the statement is insufficient. Similarly with statement 2, you could answer “yes” by again using 8, which is greater than 0, but you could find an answer of “no” by using a smaller positive number such as 4. Only when the statements are taken together must the value of x be equal to 8 (a positive number that, when squared, equals 64), and so the correct answer would be (C) BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.

You can also employ the “must” substitution when the question asks for a value. “What is the value of x?” essentially means “What must be the value of x?”. Finding two (or more) potential values of x would prove that the statement that allows you to do so is insufficient, as the question begs for one value of x which must be the case in all allowable circumstances. Put simply, in order to be “sufficient data”, a Data Sufficiency statement must be enough to guarantee one exact answer to the overall question (i.e. “yes” or “6”); if there is any wiggle room to support another answer, the statement is insufficient.

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