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.

I'm an aspie; from my own personal experience, I have to do a lot of stuff 'manually'; to make small talk, I have to actively think about it. Same thing with a lot of other activities - it doesn't just "flow", it comes from conscious effort.

It's far from "complete control over my mind", though.