GMAT Exam: A Simple Guide To GMAT Data Sufficiency Questions

About Data Sufficiency Questions
Data sufficiency questions are unique to the GMAT. In these questions, you are presented with a problem question followed by two statements containing additional information. The problem question will be either one to which you can answer yes or no or one that asks for the value of a quantity. Your task is to determine whether the data given are sufficient to answer the problem question posed. When the problem question is a yes or no question, the data given are sufficient only when it is possible to answer with a definite yes or no. In other words, you must be able to answer always yes or always no, not sometimes yes or no. When the problem question asks for the value of a quantity, the data given are sufficient only when it is possible to determine exactly one numerical value for the quantity.

Strategies for Data Sufficiency Questions
All data sufficiency questions offer you the same five answer choices.

Here is an example of a data sufficiency question.
Is x > 3? (1) (2)x > 0

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.

Use a Systematic Approach
Start by reading the question stem. The problem question is asking for a yes or no answer. Next, read statement (1). Decide whether the information in statement (1) alone is sufficient to answer the problem question. If statement (1) alone is sufficient, then A or D are the only possible answers. You can cross off B, C, and E as possibilities. On the other hand, if statement (1) alone is not sufficient, then the correct answer cannot be A or D. Consider the question given. Is statement (1) alone sufficient? (1)
The information in (1) tells you that . This statement is equivalent to the statement |x|=4. Both –4 and 4 have absolute value equal to 4. Without further information, you do not know whether x is –4 or 4. Hence, you cannot decide whether x > 3; so, statement (1) alone is not sufficient. Therefore, the correct answer is either B, C, or E.
Go on to statement (2). As you make your decision about whether statement (2) alone is sufficient, avoid considering the information given in statement (1). If statement (2) alone is sufficient, then the answer cannot be C or E. Is statement (2) alone sufficient? (2) x > 0
From the information in (2), you know that x is a positive number, but there is not enough information to determine whether x > 3. For instance, x could be a number between 0 and 3; so, statement (2) alone is not sufficient.

Now consider statements (1) and (2) together. Do they together provide sufficient information for you to answer the problem question? If they do, then the correct answer is C. If they do not, the correct answer is E.
Taking (1) and (2) together, you can determine from (1) that x is either –4 or 4, and then using (2), you can determine that x is 4. Thus, the problem answer is yes, x > 3. Therefore, BOTH statements together are sufficient, but NEITHER statement ALONE is sufficient. Choice C is the correct response.

You should approach every data sufficiency question with the same technique as illustrated in this example.

Here is a summary of the process as a decision tree.

As the decision tree shows, if statement (1) alone is sufficient, you must go on to determine whether statement (2) alone is sufficient. Don’t make the mistake of selecting A without checking statement (2).

Memorize the Answer Choices
The answer choices for data sufficiency questions do not change. The five options are always the same. Memorize them! You will save valuable time on the test by knowing them without hesitation.

Know Your Math
Having a good grasp of math will be of great help to you on data sufficiency questions. Some questions are easily answered when you have an understanding of the math concepts in the question. Also, you can avoid common pitfalls if you are knowledgeable about math concepts.

For instance, as shown in the example question, knowing that is equivalent to |x| = 4 can keep you from making the error of assuming x is 4.

Most of the math concepts on the GMAT are ones you likely have encountered in the past. However, you might have forgotten some if not most of them. 

Write Equations, But Don’t Solve Them
The equations help you organize your thinking and let you know quickly whether you have sufficient data to answer the question. When you have more than one unknown in a problem question, assign a variable name to each unknown. This strategy will help you decide whether you have sufficient information to answer the problem question.

In most cases, you will need as many linear equations as you have variables in order to determine single values for variables.

Tip: A two-variable linear equation is one that can be written as ax + by = c, where x and y are variables and a, b, and c are real numbers. Be careful though, when you have two variables and two linear equations. Make sure that the two equations are distinct equations.

For instance, x + y = 5 and 2x + 2y = 10 are different versions of the same equation. When your two-variable equations are not distinct, you will be unable to obtain a single value as an answer for a target variable. Usually, you can use visual inspection to check the two equations.

Use Your Time Wisely
Don’t solve equations or work out computations unless doing so is necessary to help you make the correct answer choice. Stop working! Click your answer choice and move on to the next question. For geometry problems, sketches help you organize the question information. Make quick rough sketches. Do not spend unnecessary time making them.

Directions for the Data Sufficiency Questions
Each problem presents a question and two statements, labeled (1) and (2), in which certain data are given. Using your knowledge of mathematics and everyday facts (such as the number of minutes in an hour or the meaning of the word perpendicular), decide whether the given data are sufficient to answer the question. Then select one of the answer choices that follow.

Note: When a data sufficiency problem asks for the value of a quantity, the data given are sufficient only when it is possible to determine exactly one numerical value for the quantity.

Also, unless otherwise stated, you can assume all of the following.
- All numbers used are real numbers.
- All figures lie in a plane.
- Lines shown as straight are straight lines, and straight lines might sometimes appear jagged.
- Figures are drawn accurately, but are NOT necessarily drawn to scale.
- All angle measures are greater than zero.
- The relative position of points, angles, and regions are in the order shown.