Solve the three most common percentage problems in seconds: what percent one number is of another, percent change, and percent of a number (discounts).
"12.5 is what percent of 50?" → 25%
"30% of 200" → 60 (works for discounts too)
"80 to 100" → +25% increase
Most everyday percentage questions come down to these three, and each is just one line:
| Scenario | Calculation | Answer |
|---|---|---|
| You got 18 out of 20 on a test. What %? | (18 ÷ 20) × 100 | 90% |
| A $150 jacket is 25% off. How much do you pay? | 150 × 0.75 | $112.50 |
| Salary rose from $50k to $55k. % increase? | (5,000 ÷ 50,000) × 100 | +10% |
| Sales dropped from 400 to 340. % decrease? | (60 ÷ 400) × 100 | −15% |
| Tip 15% on an $80 meal. Tip amount? | 80 × 0.15 | $12 |
To find 10% of anything, just move the decimal one place left. From 10%, you can build most others: 20% is double, 5% is half, and 15% is 10% + 5%. So 15% of $80 = $8 + $4 = $12. This is the same trick used by the tip calculator.
First find the discount amount: original price × (percent off ÷ 100). Then subtract it: final price = original − discount. Or shortcut: multiply the original price by (1 − percent off as a decimal). For 25% off, multiply by 0.75.
Percent change is a relative change (e.g., "rates rose 25%"). Percentage points is an absolute difference (e.g., "the rate rose from 4% to 5%, a rise of 1 percentage point"). They are often confused in news headlines, so keep them straight.
To find the original number before a percentage was applied, divide by the decimal. If a price is $90 after a 10% discount, the original is 90 ÷ 0.90 = $100.