Multiplying by 9, 99, or 999

Use these tricks to multiply quickly by 9, 99, or 999 using subtraction or complements.

Multiply by 9

Formula: a × 9 = a × 10 − a

  1. Multiply the number by 10.
  2. Subtract the original number.
43 × 9
43 × 10 = 430
430 − 43 = 387
78 × 9
78 × 10 = 780
780 − 78 = 702

Multiply by 99 or 999

Here is a different but equally simple method

  1. Subtract 1 from the number.
  2. Find the complement to 100 (for 99) or 1000 (for 999).
  3. Write the results side by side.
74 × 99
74 − 1 = 73
100 − 74 = 26
→ 7326
123 × 999
123 − 1 = 122
1000 − 123 = 877
→ 122877

Notice multiplying by 9 and 99 or 999 use two different methods but both are easy to do in a speedy manner