It looks like they were trying to fluster you. Sounds like you didn't do too badly to me. Whatever happens try to keep a positive mental attitude. This particular mental multiplication is really not that hard, but you need to have practiced in advance. The major trick in doing mental math problems is to always work left to right. If you learn no other trick, learn that one. So doing this problem, I would think to myself: twenty-one times thirty-nine is twenty times thirty nine is seven hundred eighty plus thirty nine is eight hundred ten plus nine is eight hundred nineteen. My calculator agrees that the result is 819, but for a problem like this, I could get to the mental result a tad more quickly. Here's an excellent book on doing mental arithmetic. It takes you through mentally doing addition and subtraction problems and works up to multiplication and division. With practice you will definitely be able to multiply arbitrary two digit numbers without much trouble, and depending on the time you're willing to devote to it, you will be able to do some pretty complicated problems in your head. I bet you will surprise yourself at what's possible. http://www.amazon.com/Secrets-Mental-Math-Mathemagicians-Calculation/dp/0307338401