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.

Since there are basically unlimited potential connections,
Fail. You don't have unlimited space, nor unlimited energy. Also, this is not about stored knowledge, but about processing. Neurons are just as involved as connections (signal thresholding, etc).