Multiplying two digit number by 21

1. Select a 2-digit number.
2. Multiply the number by 2.
3. Add one zero.
4. Add the original number.

Example:

1. The 2-digit number chosen to multiply by 21 is 23.
2. Multiply by 2: 2 × 23 = 46.
3. Add one zero: 460.
4. Add the original number: 460 + 23 = 483.
5. So 21 × 23 = 483.