In the quantitative section of the GMAT, questions in any subject can be presented in one of two formats: Problem Solving or Data Sufficiency.

Problem Solving (PS)

This is the “standard” format, in which a question is presented and we need to choose one option from the multiple choice options that correctly answers the question.

Data Sufficiency (DS)

In this format we are presented with a question (usually accompanied with some kind of information), which we have no way of answering. Following this, we will be provided with 2 further pieces of information referred to as statements. We are requested to decide whether the additional information provided allows us to answer the question or not. The answer choices are always:

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.

As can be seen, this format adds extra difficulty to the subjects around which the questions are built. It is therefore very important to work in a methodical and thorough manner when solving GMAT DS questions.

The following flow chart shows how to approach solving a DS format GMAT question:

GMAT_DS_QUESTIONS

An example of a DS question in the quantitative section of the GMAT:
A store owner bought Q windows at $150 per window and W shelves at $75 per shelf. What is the total cost of the windows and shelves that he bought?
(1) The Q windows cost $600
(2) Q+W/2=12
(A) Statement (1) ALONE is sufficient to answer the question, but statement (2) ALONE is not.
(B) Statement (2) ALONE is sufficient to answer the question, but statement (1) ALONE is not.
(C) Statements (1) and (2) taken together are sufficient to answer the question, even though neither statement alone is sufficient.
(D) Either statement by itself is sufficient to answer the question.
(E) Statements (1) and (2) taken together are not sufficient to answer the question, and additional data are needed to answer the question.

As established, we do not need to provide an answer for the actual question but rather decide if we are given enough information in order to be able to answer the question. We’ll start by analyzing the question:

The question can be expressed as an equation: 150Q+75W=?
Or, if we remove the common factor (75), the refined question is: 2Q+W=?

Notice that we are not being asked the values of Q and W (although if they were known, we could of course answer the question).

We’ll check the data:
The first statement tells us the value of Q but says nothing about Y. This is insufficient.
The second statement tells us that Q+W/2=12, and if we multiply this equation by 2 we’ll get 2Q+W=24, and this is the answer to our question. This is sufficient.

Since the first statement is insufficient and the second statement is sufficient, the answer is B.

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