Multiplying a 2-digit number by 198

1. Select a 2-digit number.
2. Double the number.
3. If the double has 2 digits, subtract 1:
   X X _ _
   Subtract the double from 100:
   _ _ X X
4. If the double has 3 digits, subtract 2:
   X X X _ _ (if it has 2 zeros, subtract the first digit)
   Subtract the last 2 digits of the double from 100:
   _ _ _ X X

   Example:

1. The 2-digit number chosen to multiply by 198 is 21.
2. Double it: 2 x 21 = 42.
3. Subtract 1: 42 - 1 = 41: 4 1 _ _
4. 100 - double: 100 - 42 = 58: _ _ 5 8
5. So 198 x 21 = 4158.

   See the pattern?

1. The 2-digit number chosen to multiply by 198 is 56.
2. Double it: 2 x 56 = 112.
3. Subtract 2: 112 - 2 = 110: 1 1 0 _ _
4. 100 - double's last 2 digits: 100 - 12 = 88:
   _ _ _ 8 8
5. So 198 x 56 = 11088.