Math · Live
Percentages,
solved in one tap.
Five modes that cover every percent question that actually shows up in real life — “X% of Y,” “A is what % of B,” percent change, percent increase, and percent decrease, with the formula laid out below the answer.
Inputs
Pick a mode
What is P% of N?
Live formula
15% × 200 = 30
Result
% of
Percentage applied
15%
Step by step
How we got there
- 1
Convert the percentage to a decimal
15% ÷ 100 = 0.15
- 2
Multiply by the number
0.15 × 200 = 30
- 3
Result
15% of 200 is 30
Quick table
Common percentages of 200
| Percent | Value | One-liner |
|---|---|---|
| 1% | 2 | |
| 5% | 10 | |
| 10% | 20 | |
| 15% | 30 | |
| 20% | 40 | |
| 25% | 50 | |
| 50% | 100 | |
| 75% | 150 | |
| 100% | 200 |
Field guide
What “percent” really means.
Percent comes from the Latin per centum — literally “per hundred.” A percentage is just a ratio scaled so that the whole is 100. Saying 15% is the same as saying 15 ⁄ 100 or 0.15. Once you internalise that one fact, every percentage problem collapses into either multiplication or division.
Mode 1: “What is X% of Y?”
The most-asked percentage question on Earth. Convert the percent to a decimal and multiply.
Example: 15% of 200 = (15 ⁄ 100) × 200 = 30. Useful for sales tax, tips, discounts, commissions, and almost every “take this slice off that total” question.
Mode 2: “A is what % of B?”
The reverse direction: you have two numbers and want to know their ratio expressed as a percent.
Example: 30 is (30 ⁄ 200) × 100 = 15% of 200. Use this when you're given parts and need the share — quiz scores, market share, expense breakdowns.
Mode 3: Percent change
How much something has grown or shrunk relative to its starting point. The denominator is always the original value, never the new one.
Example: From 80 to 100 is (100 − 80) ⁄ 80 × 100 = +25%. From 100 to 80 is −20%, the same absolute change, but a different denominator gives a different percent.
Mode 4: Percent increase
Add a percentage to a starting value. Common in pay raises, inflation, taxes added on top, and markups.
A $200 price increased by 15% becomes 200 × 1.15 = $230.
Mode 5: Percent decrease
Subtract a percentage from a starting value — the formula behind every “25% off” sticker on a shop window.
A $200 jacket marked down 25% rings up at 200 × 0.75 = $150.
The trap most people fall into
A 50% increase followed by a 50% decrease does not return you to where you started. Take 100, add 50% to get 150, then take 50% off — you land on 75. The percentage is applied to whatever value was current at the time, and that value keeps changing. Always ask: “percent of what?”
Mental shortcuts
- 10% of anything: slide the decimal one place left.
10% of 47.50 = 4.75. - 5%: half of 10%.
- 15% (US-standard tip): 10% plus half of itself.
- 1%: slide the decimal two places left. Useful for fast estimation: any percent equals
1% × that number. - Percentages are commutative.
8% of 50=50% of 8=4. Sometimes one direction is much easier to do in your head.
Where percentages show up
They're everywhere money is, and they show up in any field where a fraction is more readable than a decimal. Sales tax, tips, interest rates, return on investment, body-fat percentage, election results, exam grades, weather forecasts, humidity, battery level, all percentages.
Worked example: a stacked discount
A $480 sofa is on sale for 30% off, and you have a coupon for an additional 10% off the sale price. What do you pay?
- After 30% off:
480 × 0.70 = 336 - After additional 10% off:
336 × 0.90 = 302.40 - Effective discount:
(480 − 302.40) ⁄ 480 × 100 = 37%— not 40%.