375
15% of 2500
The five percentage questions people actually ask
Almost every percentage problem is one of these five. Pick the matching mode above and the formula is applied for you.
| Question | Formula | Example |
|---|---|---|
| What is X% of Y? | (X ÷ 100) × Y | 15% of 2500 = 375 |
| X is what % of Y? | (X ÷ Y) × 100 | 375 of 2500 = 15% |
| % increase | ((New − Old) ÷ Old) × 100 | 200 → 250 = +25% |
| Marks percentage | (Obtained ÷ Total) × 100 | 425 / 500 = 85% |
| Original before change | New ÷ (1 + X/100) | ₹1180 after 18% GST = ₹1000 |
Percentage increase and decrease are not symmetric
This is the mistake that costs people money. A 50% fall followed by a 50% rise does not bring you back to where you started:
- ₹1,000 falls 50% → ₹500
- ₹500 rises 50% → ₹750, not ₹1,000
The rise is calculated on the smaller number, so it recovers less. To get back to ₹1,000 from ₹500 you need a 100% increase. The same trap appears in discount stacking: "50% off, then 20% off" is a 60% total discount, not 70%.
Quick mental shortcuts
- 10% — move the decimal one place left. 10% of 850 = 85.
- 5% — half of 10%. 5% of 850 = 42.5.
- 20% — double 10%. 20% of 850 = 170.
- 15% — 10% plus 5%. 15% of 850 = 127.5.
- 1% — move the decimal two places left. 1% of 850 = 8.5.
- Flip it — X% of Y always equals Y% of X. 4% of 75 is awkward; 75% of 4 is obviously 3.
Frequently asked questions
How do I calculate a percentage of a number?
Divide the percentage by 100 and multiply by the number. 15% of 2500 is (15 ÷ 100) × 2500 = 375. Choose "What is X% of Y" above and it is done for you.
How do I work out my marks percentage?
Divide marks obtained by total marks and multiply by 100. 425 out of 500 is (425 ÷ 500) × 100 = 85%. Use the "Marks percentage" mode.
How do I find the percentage increase between two numbers?
Subtract the old value from the new one, divide by the old value, then multiply by 100. From 200 to 250 that is (50 ÷ 200) × 100 = 25% increase.
How do I remove GST from a price?
Use the "Find the original before a % change" mode with 18 as the percentage. A price of ₹1,180 including 18% GST works back to ₹1,000 before tax.
Why does a 50% loss need a 100% gain to recover?
Because the gain is calculated on the reduced amount. Losing 50% of ₹1,000 leaves ₹500, and getting back to ₹1,000 means doubling ₹500 — a 100% increase.