0580

Percentages Toolkit

Number

Two routes

  • Without a calculator, find the percentage itself, then add it to or subtract it from the original
  • With a calculator, adjust the multiplier instead and do it in a single step
  • An increase of p per cent has multiplier 1 + p/100, and a decrease has 1 − p/100
    • An increase of 12% uses 1.12
    • A decrease of 8% uses 0.92
  • The single-step route is less error-prone because the original amount is never re-added by hand
Worked example

Increasing and decreasing by a percentage

(a) Increase 640 by 12%. (b) Decrease 250 by 8%.

Solution:

  • (a) Adding 12% leaves 112% of the amount, giving a multiplier of 1.12
  • 1.12 × 640 = 716.8
  • (b) Removing 8% leaves 92% of the amount, giving a multiplier of 0.92
  • 0.92 × 250 = 230