Multiplying a 2-digit number by 999

Follow the following Steps: 

1. Select a 2-digit number.
2. Add one zero to the number, subtract 1:
   X X X _ _
3. Subtract the original number from 100:
   _ _ _ X X

   Example:1

1. The 2-digit number chosen to multiply by 999 is 64.
2. Add one zero and subtract 1: 640 - 1 = 639:
   6 3 9 _ _
3. Subtract 64 from 100: 100 - 64 = 36:
   X X X 3 6
4. So 64 × 999 = 63,936.

   Example:2

1. The 2-digit number chosen to multiply by 999 is 75.
2. Add one zero and subtract 1: 750 - 1 = 749:
   7 4 9 _ _
3. Subtract 75 from 100: 100 - 75 = 25:
   X X X 2 5
4. So 64 × 999 = 74,925.