Find a percentage of a number
Multiply the number by the percentage divided by 100. For example, 15% of 200 is 200 × 0.15 = 30. For a 15% discount on a $200 item, subtract that $30 discount from the price to get $170.
Find what percentage one number is of another
Divide the part by the whole and multiply by 100. If you answered 45 of 180 questions correctly, 45 ÷ 180 × 100 = 25%. The whole cannot be zero because division by zero is undefined.
Increase or decrease a number
A 12% increase on 250 is 250 × 1.12 = 280. A 12% decrease is 250 × 0.88 = 220. A rise and fall of the same percentage do not cancel out, because the second change uses a different starting value.
Percentages and percentage points
A rate moving from 5% to 6% rises by one percentage point. Relative to its original value, it rises by 20%, because (6 − 5) ÷ 5 × 100 = 20. Say which comparison you mean when reporting a change.