Math Shortcuts

Hello, esteemed 4chaners.  Do you have any math shortcuts that you would like to share?

For instance:

To multiply a 2-digit number by 11, take the sum of its digits. If it's a single digit number, just write it between the two digits. If the sum is 10 or more, do not forget to carry 1 over.
For example, 34x11=374 since 3+4=7. 47x11=517 since 4+7=11.

>>7
You can modify this test for any prime number. The factor of -2 is what varies from prime to prime; for example for 3 it's 1 (hence the sum the digits for divisibility by 3 rule), for 7 it's -2, for 13 it's 4, etc.

I actually calculated this for many primes back in the days when my nerd friends and I used to play prime/notprime. You say a number, they have to figure out whether it's prime or not. I don't remember many of them off the top of my head; after a while you just sortof remember the multiples of everything and you can tell whether or not a number is prime by 'intuition' or whatever.

There are a ton of variations on these tricks; here's a site full of them:
http://mathforum.org/k12/mathtips/division.tips.html