A GMAT Tutor's Tips: Treat Data Sufficiency First as a Logic Test

First-time GMAT test takers often become intimidated by the 700-800 level Data Sufficiency questions. Often, these MBA hopefuls suffer from a common fallacy for this portion of the GMAT exam.

Namely, Data Sufficiency should not be approached as a purely mathematical portion of the exam; much more often, it tests a student’s logical capabilities. As such, students should be wary of overdoing calculations when they are unnecessary. Doing so will take precious seconds off of the exam, and on occasion lead test takers to the wrong answer!

Let’s consider the following problem as a perfect illustration of what we mean: 

Sequence S has 25 numbers in it. What is the 24^th number in the sequence?

(1) Each number in the sequence is a positive, prime integer, and each is the prime number immediately following the number preceding it.

(2) The first number is even.

A skilled GMAT student knows to analyze whether the first term suffices first.

If it does, the only plausible answers are A and D; if it doesn’t, the student has narrowed the solution down to B, C, and E. Given these terms, the student will notice that the first term (1), while informative, does not give an inkling as to where the sequence of prime numbers begins. At this point, the answer must be B, C, or E, meaning the student has a 33.3% chance of guessing the answer correctly. Not a bad start!

Turning to (2), the student quickly sees that just knowing that the first number is even tells us nothing about the 24^th term in the sequence: we’re down to just C or E. Using both terms together, we know that the first term in the sequence must be 2, since 2 is the only even prime number. Thereafter, we deduce that the sequence proceeds forward from prime number to prime number (i.e., [2, 3, 5, 7 . . .]) until reaching the 24^th number in the sequence.

Once the student gets to this point, he or she should be done with the problem.

Together, both terms can answer the question. Emphasis on can. Many students make the mistake of going one step further unnecessarily. Data Sufficiency merely wants to know if the student can answer the question; it does not test what the answer actually is.

In this case, the student will count prime numbers forward to find the 24^th term: [2, 3, 5, . . . , 89] – just to prove definitively that the terms offer a discrete answer to the question. Data Sufficiency does not require a student to prove he or she can find an answer definitively.

Avoid making unnecessary calculations once you know that the terms suffice, and you will save precious time for the tougher questions that follow! If you take shortcuts during your standardized test preparation, you will save yourself pain on test day!