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Why am I better at GMAT Problem Solving than Data Sufficiency?

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Manhattan Prep GMAT Blog - Why Am I Better at GMAT Problem Solving than Data Sufficiency? by Chelsey Cooley

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The next time you review a practice GMAT, pay attention to the difference between Problem Solving and Data Sufficiency. Are you missing significantly more Data Sufficiency questions than Problem Solving questions? Or are you missing an unusual number of easy DS questions?

This can happen for a few different reasons, depending on where you are in your studies. If you’re just getting started, you may not have a strong grasp on the Data Sufficiency process. Try using this approach strictly, every single time you do a Data Sufficiency problem.

  1. Read the question, and do three things.
    1. Decide whether it’s a ‘yes/no’ question or a ‘value’ question. Yes/no questions, such as ‘is x greater than 15?’, can be answered with a simple ‘yes’ or a ‘no’. Value questions, such as ‘what is the value of x?’, can only be answered with an exact number.
    2. Write down any facts the question gives you. If the question starts with ‘If x is a positive integer’, then jot that down on your paper.
    3. Translate and simplify the question, if necessary. If it’s a word problem, always write it out in mathematical terms. Once you’ve got that done – or if the problem gives you an algebraic expression to start with – simplify it as much as possible using algebra. Don’t start working with the statements until the question is in the simplest possible form.
  2. Choose a statement to start with. If one statement looks easier than the other, always begin with that one.
    1. Translate and simplify the statement, if necessary. If the question was a word problem, then translate the statement using the same variables.
    2. Is the statement obviously sufficient? (That is, does it translate to a single, exact answer to the question?) If so, move on to the next statement.
    3. If it isn’t obvious, or if you think the statement is insufficient, try to prove it. In order to prove that a statement is insufficient, you have to show that even with the information from the statement, the question could still have multiple correct answers.

If you’re somewhere in the middle of your studies, you may already be using this process. But if you’re still missing DS problems, ask yourself these questions.

  1. Are you really proving that the statement is insufficient before you move on? Many problems include statements that look insufficient, but actually aren’t. Don’t just assume! Actually prove that the question could have two different answers. And if that turns out to be harder than you anticipated, maybe the statement is sufficient after all!
  2. Are you falling for C traps? If both statements together are sufficient, that doesn’t mean the answer is C! C is only correct if both statements are insufficient alone, and they’re sufficient together. Both pieces have to be true for C to be right.
  3. Are you making any of the type 1 or type 2 Data Sufficiency errors? Look through the DS problems from your last practice test, and decide why you made each mistake. Some of your wrong answers might have been unlucky guesses, or mathematical errors. That’s fine – you make the same kind of mistakes on Problem Solving! But if you’re making any of the classic Data Sufficiency mistakes, identify and address them.

If you really want to take your Data Sufficiency performance to the next level, try this exercise. It isn’t easy, but it’ll give you deep insight into this problem type.

  1. Go to your latest practice test, or a long problem set that included multiple Data Sufficiency questions. If you haven’t done one lately, now is the time! Try doing a set of 20 random DS problems from the Official Guide to the GMAT, 2016 or from the GMAC’s GMATPrep software.
  2. For each Data Sufficiency question, regardless of whether you got it right, do the following:
    1. Check the right answer, and figure out which statements are sufficient and which aren’t.
    2. For each sufficient statement, write down exactly how to determine the single right answer to the question, based on that statement. That could involve algebra, logic, or both.
    3. For each insufficient statement, come up with two different cases, or scenarios, that fit the statement. Using math, prove that each case leads to a different answer to the question.

This will probably be tougher than you think! Here’s an example of how you might attack an insufficient statement.

If x and y are different positive integers, what is the value of x2 + y2?

  1. xy = 36

In your notes, you’d write out the following:

Case 1: x = 1, y = 36. Answer = x2 + y2 = 12 + 362 = 1297

Case 2: x = 3, y = 12. Answer = x2 + y2 = 32 + 122 = 153

That’s much more than you’d actually do on test day, so it’s okay if it takes more than two minutes! Doing it when you review will force you to look deeply into exactly why the statement is insufficient – and will also give you a great chance to practice mental math. ?

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Chelsey CooleyChelsey Cooley Manhattan Prep GMAT Instructor is a Manhattan Prep instructor based in Seattle, Washington. Chelsey always followed her heart when it came to her education. Luckily, her heart led her straight to the perfect background for GMAT and GRE teaching: she has undergraduate degrees in mathematics and history, a master’s degree in linguistics, a 790 on the GMAT, and a perfect 170/170 on the GRE. Check out Chelsey’s upcoming GMAT prep offerings here.