Multiplying a 2-digit number by 81

1. Select a 2-digit number.
2. Multiply it by 9.
3. Add one zero.
4. Subtract the first number from the second.

   Example:

1. The 2-digit number chosen to multiply by 81 is 31.
2. Multiply by 9: 9 x 31 = 279
3. Add one zero: 2790
4. Subtract: 2790 - 279 = 2511
   (Think: 2790 - 200 - 70 - 9 = 2590 - 70 - 9 = 2520 - 9 = 2511)
5. So 31 x 81 = 2511.

   See the pattern?

1. The 2-digit number chosen to multiply by 81 is 68.
2. Multiply by 9: 9 x 68 = 612
   (Think: 9 x 6 + 9 x 8 = 540 + 72 = 612)
3. Add one zero: 6120
4. Subtract: 6120 - 612 = 5508
   (Think: 6120 - 600 - 10 - 2 = 5520 - 10 - 2 = 5510 - 2 = 5508)
5. So 68 x 81 = 5508.

Practice subtracting mentally by doing it in steps from left to right. With that skill you will be able to multiply by 81 quickly.