X% of Y = Y × X ÷ 100
"Percent" means "per hundred", so 35% is 35 out of every 100, or 0.35. To take 35% of 80, multiply 80 by 0.35: 28. The same idea works in reverse (what percent is 40 of 60?) and for discounts, tips and tax.
How to find the percentage of a number, step by step
Turn the percent into a decimal by moving the point two places left (35% → 0.35). Multiply: 80 × 0.35 = 28. Done. If you prefer fractions, 35% of 80 is 35 ÷ 100 × 80.
How to find what percentage one number is of another
Divide the part by the whole and multiply by 100. 40 is what percent of 60? 40 ÷ 60 = 0.667, so 66.7%. What is 40/60 as a percent? The same: 66.7%.
How to find the percentage of two numbers
Usually this means the same thing: the first as a percentage of the second. 15 out of 20 is 15 ÷ 20 × 100 = 75%.
How do you do percentages off?
Work out the percentage, then subtract, or multiply by (100 − percent)%. 20% off $50: $50 × 0.80 = $40. 15% off $80: $80 × 0.85 = $68.
Mental shortcuts
10% is the number with the decimal moved one place (10% of 80 = 8). 5% is half of that (4). 1% is two places (0.8). Build any percent from those: 35% = 3 × 10% + 5% = 24 + 4 = 28.