One less than 100
How to multiply by 99 mentally
Ninety-nine is one less than 100. Create 100 copies of the number, then remove one copy. The round intermediate value makes the subtraction easier to see and check.
The pattern
n × 99 = (n × 100) − n
Step by step
- 1Multiply the number by 100.
- 2Subtract the original number once.
- 3Check that the answer is slightly smaller than the ×100 result.
Worked examples
8 × 99
800 − 8
792
24 × 99
2,400 − 24
2,376
125 × 99
12,500 − 125
12,375
The same compensation idea works near other round numbers: ×19 can become ×20 minus one copy, and ×101 can become ×100 plus one copy.
Try the pattern
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