percent
The word percent comes from Latin for “per hundred.” A percent is a ratio whose second quantity is fixed at 100, which makes different fractions easy to compare on one familiar scale. Saying 37 percent simply means 37 out of every 100, written 37%.
Every percent is also a fraction and a decimal in disguise. To convert a percent to a decimal, divide by 100 (move the decimal point two places left): 37% = 0.37 and 5% = 0.05. To go the other way, multiply by 100. The fraction form of 37% is 37/100, which may then reduce; 50% = 50/100 = 1/2.
To take a percent of an amount, multiply: 20% of 80 is 0.20 × 80 = 16. Two cautions are worth remembering. Percentages above 100% are perfectly valid (150% of 80 is 120, more than the original), and a percent increase followed by the same percent decrease does not return you to the start — a 10% rise then a 10% fall leaves you slightly below where you began.
A 60-dollar shirt is marked 25% off. The discount is 0.25 × 60 = 15, so the sale price is 60 − 15 = 45 dollars.
Convert the percent to a decimal, multiply to find the part, then adjust the whole.
Beware the difference between percentage points and percent. Going from 4% to 6% is a rise of 2 percentage points, but a 50% increase relative to the original 4%. Headlines often blur these two.