Data Sufficiency problems make up about twofifths of the quantitative questions on the GMAT. They are not direct questions which ask to give the answer to the question. They ask to determine whether or not you can answer the question with the information given.
Data sufficiency questions don’t actually require you to solve the problem. Instead, you have to evaluate two statements and determine which of those statements provides sufficient information for you to answer the question.
A problem and two statements (1) and (2) are given in the question. All Data Sufficiency questions have following five choices:
 Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
 Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
 BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
 EACH statement ALONE is sufficient.
 Statements (1) and (2) TOGETHER are NOT sufficient.
It’s possible that just one of the statements gives enough data to answer the question, that the two statements taken together solve the problem, that both statements alone provide sufficient data, or that neither statement solves the problem, even with the information provided by the other one.
How to Solve Data Sufficiency Problems
Take a methodical approach to answer datasufficiency questions, and follow this series of steps:
 Evaluate the question to make sure you know exactly what you’re supposed to solve, and, if you can, decide what kind of information you need to solve the problem.
 Examine one of the statements and determine whether the data in that one statement is enough to answer the question.
 Examine the other statement and determine whether it has enough information to answer the question.
 Examine the statements together to determine whether the data given in both is enough information to answer the question.
Tips for Data Sufficiency Problems

In problem solving questions, the focus is to find the answers. However, in data sufficiency this is not the case. The focus here is whether you can find the answer or not? You only need to determine whether sufficient information is given to solve it.

You can memorize the answer choices. Though through practice this will be automatic process. There are five answer choices which are always same.

In Data Sufficiency, you have to first consider each statement separately. Only if both the statements alone are not sufficient, you should consider the sufficiency of the information in the combined statements.