Formulas
| Question | Formula | Example |
|---|---|---|
| What is X% of Y? | Y × X ÷ 100 | 15% of 80 = 12 |
| X is what % of Y? | X ÷ Y × 100 | 12 is 15% of 80 |
| % change from A to B | (B − A) ÷ |A| × 100 | 80 → 100 = +25% |
| Price after X% off | Price × (1 − X ÷ 100) | $80 with 25% off = $60 |
Percent vs percentage points
If an interest rate goes from 4% to 5%, it rose by 1 percentage point, but by 25 percent (because 1 is 25% of 4). News stories mix these up all the time.
Tip: reverse a discount
To find the original price from a sale price, divide instead of multiplying: $60 after 25% off came from $60 ÷ 0.75 = $80.
