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.

Divisible by 7 test:
1. Remove the last digit, double it, and subtract it from the remaining digits.
2. If the result is negative and there are 2 or more digits, drop the negative sign.
3. Repeat until you end up with a multiple of 7. (-7, 0, or +7)

1358 => 135 - (8*2) = 119 => 11 - (9*2) = -7
1358/7 = 194