Multiplying two digit number by 23

1. Select a 2-digit number.
2. Add two zeros to number.
3. Divide by 4.
4. From this subtract original number twice.

Example:

1. First pick is 21.
2. Add two zeros: 2100.
3. Divide by 4: 2100/4 = 525.
4. Subtract original number twice: 525 - 21 - 21 = 504 - 21 = 484 - 1 = 483.
5. So 23 × 21 = 483.