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.

When multiplying a numver by 5, multiply it by 10 and then divide it by 2.

Or, and this is easier, first divide it by 2, then move the decimal point over one (multiply by 10). This is the quickest way to do it in your head

236 * 5 = 118 * 10 = 1180

I know this is pretty simple, but I figured it out in 4th grade, but my math teacher wouldn't believe me and totally discredited it. Since then I've always used it, constantly.