The formulas, in case you want to do it by hand
X% of a number: number × (X ÷ 100).
What % one amount is of another: (amount ÷ total) × 100.
Percentage change: ((final value − initial value) ÷ initial value) × 100. Positive is an increase, negative is a decrease.
Common mistake: a 50% increase followed by a 50% decrease does NOT bring you back to the original number. Example: 100 goes up 50% → 150; then down 50% → 75, not 100. Percentages aren't symmetric when the base they're calculated on changes.