X% of Y = X × Y ÷ 100. Percent change = (new − old) ÷ old × 100.
Three common questions
What is X% of Y? Multiply: 15% of 80 = 0.15 × 80 = 12. X is what percent of Y? Divide: 12 ÷ 80 = 15%. Percent change? (new − old) ÷ old: from 80 to 92 is +15%.
Percent vs percentage points
If a rate goes from 4% to 5%, it rose 1 percentage point but 25 percent. Loan and savings rates are usually compared in percentage points. A 50% drop followed by a 50% rise does not get you back to the start: 100 → 50 → 75.
Frequently asked questions
What is 15% of 80?
12.
How do I add 20% to a price?
Multiply by 1.20.
How do I remove 20% that was already added?
Divide by 1.20, not multiply by 0.80.