MEPX
Chapter 6 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 6 of 11

Percentages

Everything is built from ten percent.

Start at ten

Ten percent is the number with the decimal point moved one place left. Five percent is half of that, and one percent is two places instead of one.

  • Fifteen percent of 240 is 24 plus 12, which is 36.
  • A twenty percent tip is ten percent doubled.

Reversing them

Percentages commute: 8 percent of 50 is the same as 50 percent of 8, which is 4. When one direction looks hard, try the other way round.