The GMAT's data-sufficiency questions don't necessarily require you to calculate a specific mathematical answer; they require that you recognize whether a specific problem could be answered with the information provided. These problems usually take less time than problem- solving questions.

Data-sufficiency questions test your ability to analyze a problem, to recognize relevant or irrelevant information in determining the solution of that problem, and to determine when you have sufficient information to solve that problem.

Correctly answering these questions requires competence in high school arithmetic, algebra, and intuitive geometry. Mathematical insight and problem-solving skills are also necessary. No advanced mathematics is required.

Here's a sample question:

What is the area of circle O?

1. The circumference is 12π.

2. The diameter is 12.

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 (1) and (2) together are sufficient, but neither statement alone is sufficient.

D. Each statement alone is sufficient.

E. Statements (1) and (2) together are not sufficient.

To find the area of a circle, it is necessary to have the radius. (1) gives you enough information to find the radius by substituting into the circumference formula, C = 2πr, and getting 12π  = 2πr. Then simply solve for r, which is 6. Thus the area is 36π. None of this is necessary, only knowing that you need the radius and can find it from the information given. (2) also gives enough information to find the radius; therefore answer is D, either will be sufficient.

Here is one more sample question:

If 2x + 3y = 15, then what is the value of x?

(1) y = x + 2

(2) y is a prime number less than 7.

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 (1) and (2) together are sufficient, but neither statement alone is sufficient.

D. Each statement alone  is sufficient.

E. Statements (1) and (2) together are not sufficient.

To solve for two variables, you need two equations containing those variables or information that will give you a value for one of the variables.

The first bit of data gives you that second equation, so you now have two equations containing the two variables. You can find a value for x.

The second bit of data does not give you a value for y, it simply limits is to 2, 3, or 5. So you cannot solve for a value of x. The correct answer is A.