Mental maths strategies

Mental maths means working things out in your head using clever shortcuts. The trick is to change the question into an easier one that gives the same answer.

Key facts

  • Partition: split numbers into tens and ones.
  • Compensate: + 99 is + 100 − 1.
  • Double and halve: 25 × 16 = 50 × 8 = 100 × 4.
  • Use facts you know: if 6 × 7 = 42, then 60 × 7 = 420.

Worked examples

Example 1

Work out 346 + 99.

  1. Adding 99 is adding 100 then taking 1 away.
  2. 346 + 100 = 446
  3. 446 − 1 = 445

Answer: 445

Example 2

Work out 25 × 16.

  1. Double 25 and halve 16. 25 × 16 = 50 × 8
  2. Do it again. 50 × 8 = 100 × 4 = 400

Answer: 400