Some GMAT questions will introduce the concept of interest earned on a Principal. These questions are easily identified by the use of the term "Interest" somewhere in the question (Duh!).
We recognize two different kinds of interest - Simple interest, and Compound interest. Look for the following three pieces of the puzzle in the question:
1) Interest rate - usually given as a %
2) Time - How long the interest is calculated for.
3) Compounded - Annually, semi-annually (every six months), quarterly, monthly.