Math · Quick Answer
How do you calculate a percentage?
Percentage = (part / whole) × 100. To find X% of a number, multiply the number by X/100. Example: 20% of 150 = 150 × 0.20 = 30.
The three common percentage problems
1. What is X% of Y?
(X/100) × Y
Example: What is 15% of 80? → 0.15 × 80 = 12
2. X is what percent of Y?
(X / Y) × 100
Example: 12 is what percent of 80? → (12/80) × 100 = 15%
3. X is Y% of what?
X / (Y/100)
Example: 12 is 15% of what? → 12 / 0.15 = 80
Percentage increase / decrease
% change = ((new − old) / old) × 100
Example: Price rises from $80 to $92. Change = (92 − 80)/80 × 100 = 15% increase.
Quick mental math tricks
- 10%: move the decimal one place left
- 5%: take 10% and halve it
- 20%: double the 10%
- 15%: 10% + 5% (10% + half of 10%)
- 1%: move decimal two places left