Math tricks and shortcuts

Early Learning Teaching Your Child Math
1

I’ve always been partial books, tips and tricks to improve mental calculations. Just found this one:

http://wildaboutmath.com/2007/11/04/how-to-square-large-numbers-quickly-part-1/

2

To multiply numbers less than 20

  1. Take the higher number and add the “ones” digit to this number
  2. Add a zero to the end(multiply by ten)
  3. Multiply both the digits in the “ones” place by eachother
  4. Add this to #2.

Example.
15 x 17

  1. 17(higher number) plus 5 (ones digit of the lower number) =22
  2. 22–> 220
  3. 5 x 7=35
  4. 220 + 35 =255