Mathematical trick

Trick 2 : Slide Up or Down
You may remember from school, that subtraction was sometimes referred to as the difference between two numbers so when calculating numbers in my head, I sometimes slide BOTH numbers up or down in my head (which keeps the difference between them the same):

 52
-19

I think, if I slide the 19 up one to 20, then slide the 52 up one to 53, it becomes:

  53
- 20
  33

This is easier to do in your head (no carries to think about)

Another example  84 - 68:

  84
- 68

I think, if I slide the 84 up two to 86, then slide the 68 up two to 70, it becomes:

  86
- 70
  16

Again, no carries to think about. Prove this to yourself by calculating 84 - 68 = 16