MEPX
Chapter 4 of 11All chapters
  1. 1Why bother
  2. 2Splitting numbers
  3. 3Rounding and compensating
  4. 4Doubling and halving
  5. 5Shortcuts worth knowing
  6. 6Percentages
  7. 7Fractions and decimals
  8. 8Squares and roots
  9. 9Division
  10. 10Estimation and checks
  11. 11Building speed

Chapter 4 of 11

Doubling and halving

Move a factor of two across the multiplication.

The trick

Doubling one factor and halving the other leaves the answer unchanged. 25 times 16 becomes 50 times 8, then 100 times 4, which is 400.

  • Repeat until one side is a power of ten.
  • It works in the other direction too: 14 times 5 becomes 7 times 10.

Halving fractions

Halving a fraction doubles the number underneath. Half of three quarters is three eighths, and half of two thirds is one third.