How we count
One percent is one hundredth: p % of a number is p ÷ 100 × the number.
A percentage change compares the difference with the starting value: (new − old) ÷ old × 100. That is why +20 % and then −20 % does not bring you back to where you started.
After a discount of p % you pay (1 − p ÷ 100) of the price, so the original price is the paid price ÷ (1 − p ÷ 100) — not the paid price plus p %.
Questions
- How do I calculate 15 % of 80?
- 15 ÷ 100 × 80 = 12. Pick “X % of Y” and enter 15 and 80.
- How do I work out a percentage increase?
- Subtract the old value from the new one, divide by the old value and multiply by 100: from 80 to 100 is (100 − 80) ÷ 80 × 100 = 25 %.
- What was the price before a 25 % discount?
- Divide the price you paid by 0.75: 60 ÷ 0.75 = 80. Adding 25 % to 60 would give 75, which is wrong.