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GMAT Data Sufficiency Tips

GMAT Data Sufficiency: What it is, in a nutshell

GMAT Data Sufficiency is a question type on the GMAT Quantitative section. On the GMAT, the format of each Data Sufficiency problem is the same: you’re asked a question and your’e given two mathematical statements. You then decide whether the statements give you enough information to answer the question. You don’t need to give the answer to the actual question. You just have to decide whether either statement (or both statements) give data that is sufficient for finding an answer.

Tips for Solving GMAT Data Sufficiency Problems

On probably every standardized test you have taken, there have been multiple choice math problems. That’s exactly what the Problem Solving questions are.  No big surprise there.  The other type of question, though, Data Sufficiency, is a beast unique to the GMAT.  I would argue that the Data Sufficiency section is extraordinarily apt for the GMAT, as it tests uniquely managerial skills.  On the GMAT Quantitative section, you get 75 minutes for 37 questions —- of these 37 questions, approximately 14-16 will be Data Sufficiency questions.

If you are just encountering this question for the first time, here are some tips that will help you negotiate this often confusing question format. When you’re done having a look at them, check out our GMAT eBook to help you navigate the rest of the test.

 

GMAT Data Sufficiency Tip #1: memorize the answer choices

The five answer choices are always the same.  Know them cold.  If someone breaks into your residence at night, wakes you with a bucket of ice water, and demands that you recite the five DS answer choices, you should be able to rattle them off with no problem.

Here are those five answer choices (exact wording).

  1. Statement 1 alone is sufficient but statement 2 alone is not sufficient to answer the question asked.
  2. Statement 2 alone is sufficient but statement 1 alone is not sufficient to answer the question asked.
  3. Both statements 1 and 2 together are sufficient to answer the question but neither statement is sufficient alone.
  4. Each statement alone is sufficient to answer the question.
  5. Statements 1 and 2 are not sufficient to answer the question asked and additional data is needed to answer the statements.

 

GMAT Data Sufficiency Tip #2: use elimination and guessing to your advantage

On a Problem Solving question, if you eliminate one answer, the other four are still there.  While guessing the answer from among four choices is slightly better odds than guessing from among five, this is not much of a gain.

Fortunately, you can often eliminate answers two or three at a time on the Data Sufficiency.  Suppose you can make a sensible decision about one of the two statements, and the other statement is completely incomprehensible to you.  Here’s what you can do:

Case One: Statement #1 is sufficient on its own, Statement #2 is incomprehensible

Eliminate: B, C, E

Only possible answers: A, D

Case Two: Statement #1 is not sufficient on its own, Statement #2 is incomprehensible

Eliminate: A, D

Only possible answers: B, C, E

Case Three: Statement #1 is incomprehensible, Statement #2 is sufficient on its own

Eliminate: A, C, E

Only possible answers: B, D

Case Four: Statement #1 is incomprehensible, Statement #2 is not sufficient on its own

Eliminate: B, D

Only possible answers: A, C, E

In all four cases, you can eliminate at least two answers, so even if you guess randomly from the remaining answers, the odds are very much in your favor.

 

GMAT Data Sufficiency Tip #3: focus on the sufficiency question

On Problem Solving questions and in most traditional math, the focus is: find the answer.  That’s not the focus in Data Sufficiency.  The focus in DS is: could you find the answer? In other words, do you have enough information to be able to find the answer?  That’s the sufficiency question, and which answer choices you choose or eliminate depends on answers you find to the sufficiency questions for each statement.

The prompt for a Data Sufficiency problem is always a question, either “find the value of blah blah blah” or “is x blah blah blah”, either a “value” question or a “yes-no” question.  Especially if the prompt is a “yes-no” question, do not confuse the answer to the prompt question with the answer to the sufficiency question.

Let’s consider a very easy example to demonstrate this difference.

1.  Is x > 5?

Statement #1: blah blah blah

Statement #2: x = 3

This question is way too easy to appear on the GMAT, but let’s consider it.  Statement #2 tells us, definitively, that x = 3.  Therefore, we can give a clear and resounding “no” answer to the prompt question.  But that is NOT the answer to the sufficiency question.  If we are in a position to give a clear and definitive answer to the prompt question — any answer, as long as it’s clear and certain — then the information must be sufficient. Therefore the answer to the sufficiency question is “yes”, regardless of whether the definitive answer to the prompt question is “yes” or “no.”  By contrast, a “no” answer to the sufficiency question means something very different — it always means we were not able to determine anything definitive at all about the prompt. That is, you can’t get any clear answer to the prompt question.  If you can get an answer to the prompt, then the sufficiency answer is “yes.”

 

GMAT Data Sufficiency Tip #4: don’t calculate an exact answer if you don’t have to

This is a continuation of the previous tip.  Data Sufficiency is all about — “could you find the answer?”  Suppose the prompt is “What is the value of x?”, a standard DS problem.  Suppose, in the course of solving this problem, you get to a step like 23x + 144 = 5670.  The benighted student with poor managerial instincts will dutifully work through the several steps necessary for finding the actual value of x — without a calculator.  The skilled GMAT test taker would realize: “From the step 23x + 144 = 5670, I could solve for x. That, in and of itself, answers the sufficiency question right there, and in answering the sufficiency question, I am done. The actual value of x is irrelevant to answering the question.”

By the way — this idea has some profound applications in Euclidean geometry.

 

GMAT Data Sufficiency Tip #5: consider the statements separately first

On the Data Sufficiency, you have to first consider each statement separately —- consider whether each statement, by itself, is sufficient.  Only if both statements are not sufficient separately would you consider the sufficiency of the information in the combined statements.

One of the biggest mistakes folks make in the Data Sufficiency format is to read Statement #1, do some calculations or deduction, and then carry all that information with them when they consider Statement #2.  This is particular tempting if Statement #1 contains all kinds of juicy information with fascinating logical implications.  People get wrapped up in the “what if Statement #1 is true” world, and they have trouble leaving that world behind when it comes time to pay attention to statement #2 alone.

One helpful strategy: consider whichever statement is simplest first.  The GMAT loves making Statement #1 a huge, complicated, juicy statement and Statement #2 something incredibly brief.  If that’s the case, consider Statement #2 first.  You have to consider the two statements separately, but there’s no law saying in which order you need to consider them.  Choose the simpler one first.

 

GMAT Data Sufficiency Tip #6: be smart about picking number

Many people, before studying for the GMAT, haven’t thought about math for a while.  For these folks, it’s often the case that if the problem uses the word “number”, their minds default to a very limited set.  Often, that set consists of only the natural numbers, a.k.a the counting numbers —- {1, 2, 3, 4, …}.  People forget that a “number” could be positive or negative or zero, could be a fraction, could be a square root, could be pi or some other decimal, etc. etc.   The possibilities are literally infinite.

The GMAT loves to test number properties, and one of the principle reasons why is that people without number sense make all kinds of predictable mistakes.  The statement might say, for example, x > 7, and people will read that and assume in hordes that x must be 8 or more, totally forgetting that there is an entire continuous infinity of decimals between 7 and 8.  When picking numbers for variables, people predictably pick positive whole numbers and predictably forget negative integers, positive fractions, negative fractions, etc. etc.   Be smart about picking numbers on Data Sufficiency.  Always have absolutely every kind of number in mind when you are analyzing a Data Sufficiency question.

 

Here’s a practice question from inside the Magoosh product:

2. http://gmat.magoosh.com/questions/928

 

Let us know below how you do!

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