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.

if your half decent at being able to remember a list of numbers in your head and gd at mental arithmatic there is a shotcut on long multiplication

break up each part of the sum into its units, tens, 100's, etc then multiply that by all of the others.

347*134= (300*100)+(300*30)+(300*4)+(40*100)+(40*30)+(40*4)+(7*100)+(7*30)+(7*4)
=30,000+9,000+1,200+4,000+1,200+160+700+210+28
=46,498
ok that may look horrid but i can add that in my head without having to write anything down

might not be shorter if you have pen and paper but if you got a gd memory then you can do it without.

a temp maths teacher taught this when i was 10, then i used to confuse the crap out of the highschool teachers.