Quote from Pathus:
Example: "Tell me the hardest thing you have ever done in your life."
Me: "Well, I ... Them: "WHAT IS 21 TIMES 39?"
[...]
It is a top propriety trading firm. The thing I can't do that well is
multiply numbers in my head. I am aware that there are a lot of easy tricks
to do math mentally, but I do not know them yet.
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