Making 100
For 100 minus a number, the tens go up to 9 and the units up to 10.
100 − 37 = 63
- 9 − 3 = 6
- 10 − 7 = 3
- 60 + 3 = 63
Mental math techniques
Tricks to calculate faster in your head, sorted by level. Each one comes with a worked example, step by step.
Level 2 · Medium 5 techniques
Subtracting by compensation
Add the same number to both terms to land on a round ten: the difference stays the same.
73 − 28 = 45
- 73 + 2 = 75
- 28 + 2 = 30
- 75 − 30 = 45
Multiplying by 5
× 5 is × 10, then half.
48 × 5 = 240
- 48 × 10 = 480
- 480 ÷ 2 = 240
Multiplying by 9
× 9 is × 10 minus the number once.
17 × 9 = 153
- 17 × 10 = 170
- 170 − 17 = 153
Multiplying by 11
For a two-digit number, spread its digits apart and slip their sum in the middle.
43 × 11 = 473
- 4 + 3 = 7
- 400 + 70 + 3 = 473