Yesterday, we posted a GMAT practice question on Facebook. It got a lot of attention and many responses. Here it is again:

**What is the value of b if a = ?**

Our Facebook audience mostly answered A, with a few votes for D. **The correct answer is indeed A**, and here’s why…

First, you can multiply both sides of the equation in the question stem by to make it clear that . Then, look for values a, c and .

**Statement (1) is exactly what is needed**– it gives you a precise value for . Statement (1) is sufficient, so eliminate answer choices C and E.- Statement (2) alone, however, leads to two possible values for b, because you’d have to substitute the square roots of 25 for a, and those square roots are BOTH positive AND negative 5 (remember this! The GMAT likes to test you on your knowledge that both a positive and negative number can be squared to produce a positive result.) Therefore, Statement (2) is insufficient because it does not give you one single value. Eliminate answer choices B and D.

The correct answer is A.

Questions about data sufficiency? Topics you’d like to see covered in future blog entries? Talk to me in the comments!