Multiplying two digit number by 29

1. Multiply by 3.
2. Add one zero.
3. From this, subtract the original number.

Example:

1. The 2-digit number chosen to multiply by 29 is 21.
2. Multiply by 3: 3 × 21 = 63
3. Add one zero: 630
4. Subtract original number:
   630 - 21 = 610 - 1 = 609
5. So 21 × 29 = 609.