Regrouping factors
In a chain of multiplications the order does not matter: bring together the ones that make a round number.
25 × 7 × 4 = 700
- 25 × 4 = 100
- 100 × 7 = 700
Mental math techniques
Tricks to calculate faster in your head, sorted by level. Each one comes with a worked example, step by step.
Level 4 · Expert 4 techniques
Around a round number
Two numbers the same distance from a round number: (m − d) × (m + d) = m² − d².
48 × 52 = 2496
- 50 × 50 = 2500
- 2 × 2 = 4
- 2500 − 4 = 2496
Multiplying by 99
× 99 is × 100 minus the number once.
23 × 99 = 2277
- 23 × 100 = 2300
- 2300 − 23 = 2277
Dividing by 25
There are four 25s in 100: multiply by 4, then divide by 100.
475 ÷ 25 = 19
- 475 × 4 = 1900
- 1900 ÷ 100 = 19