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Data Sufficiency

GMAT math data sufficiency
Data Sufficiency (DS) questions use the same math concepts as Problem Solving questions, but ask a unique type of question that many students initially find confusing. DS questions ask you a question, often accompanied by some initial information, followed by two statements labeled (1) and (2) which contain additional information.

Your objective is to determine whether statements (1) and/or (2) separately or combined provide enough (i.e. sufficient) information to derive one definitive answer to the initial question. Other than the information explicitly provided, you cannot assume anything besides common knowledge facts (e.g. there are 60 minutes in an hour).

Answer Choices on GMAT Data Sufficiency

DS questions always present the same five answer choices. To save time, memorize these answer choices. A helpful acronym by which to memorize the answers is 12TEN.

        (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.

In evaluating sufficiency, first consider the information only in (1). Then consider the information only in (2), ignoring the information in (1) that you just evaluated. Only if (1) by itself is not sufficient and (2) by itself is not sufficient, then evaluate sufficiency using (1) and (2) in combination. Consider the following answer elimination flowchart:

Data Sufficiency flowchart

How to Approach GMAT Data Sufficiency

When evaluating sufficiency, you must determine whether it is possible to derive only one definitive answer to the question asked. There are two broad types of DS questions: numerical value and Yes/No.
  • For numerical value questions (e.g. What is the value of x?), you must be able to derive only one specific value (e.g. x = 37) to have sufficient information. If you could come up with more than one value or a range of values, you do not have sufficient information.
  • As the name implies, Yes/No questions (e.g. Does x = 37?) can be answered only with a Yes and/or a No (e.g. Yes, x does equal 37). For these questions, you must be able to answer definitely Yes or definitely No to have sufficient information.
There are two broad approaches to DS questions.
  • The often-faster, more theoretical approach entails relying on a mathematical justification. For example, an algebraic principle is that a set of unique linear equations is solvable if the number of equations equals the number of variables. In this case, you do not actually have to solve out the equations. Merely knowing that you could solve them and derive a unique solution is all that matters.
  • A second approach entails plugging in numbers. Try different numbers that meet any given constraints to assess whether those different numbers could yield different answers or always yield the same answer to the question asked.

Sample GMAT Data Sufficiency Problem

Let’s try a sample problem. Attempt the problem on your own before viewing the answer and explanation.

If a, b, and c are positive integers, is (a + c)(b + c) an even integer?

        (1) a is odd
        (2) b is even

See above for answer choices.

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