There are several general guidelines to help you determine when you can approximate on a GMAT math problem:
1. More problems can be solved by approximation than you think.
2. When you round up or down, make a note of which way you’ve adjusted.
3. If you’re multiplying two numbers that are each close to a round number, but differ from nearby round numbers in opposite directions (like 11 and 19, or 9 and 21), using the round numbers will get you very close.
4. The answer choices are a major hint to whether approximation is appropriate. If the answer choices are spread out, approximate away!