Anyone got any good mental math tricks to share? This one is pretty rarely useful but it's cool when it is.

When multiplying 2 numbers with the same digit in the tens column and digits in the ones column that add up to to 10 (eg. 87 * 83)

You can :

Multiply the tens column together, adding one to one of them beforehand
8*(8+1)= 72

Multiply the ones column digits:
7*3=21

And then place the numbers next to each other for the answer
=7221
Wa la