IGCSE Maths 0580 · Common mistakes
Common percentage mistakes in IGCSE Maths
Updated · by the Fahmazing team
Percentages appear on almost every IGCSE Maths paper, often hidden inside money, profit or growth questions. The same few slips come up again and again. Here they are, with the fix for each.
1. Forgetting to divide by 100
A percentage under 100% of an amount must be smaller than the amount. If your answer is bigger, you have forgotten to divide by 100.
Find 35% of 200
✗ Wrong: 200 × 35 = 7000, or 700
✓ Right: 200 × 35 ÷ 100 = 70. Check: 70 is about a third of 200. Sensible.
Free lesson: Percentages
2. Giving the discount instead of the new price
Reread what the question asks. "How much is the discount?" and "How much does it cost now?" are different answers.
A jacket costs 80 rials. It is reduced by 15%. What is the sale price?
✗ Wrong: 10% = 8, 5% = 4, so 15% = 12 rials
✓ Right: Discount = 12 rials, so sale price = 80 − 12 = 68 rials
Free lessons: Percentages · Money
3. Using the wrong value as the whole
Percentage change and percentage profit are always out of the original amount (the cost price), never the new one.
A trader buys a bike for 40 rials and sells it for 50 rials. Find the percentage profit.
✗ Wrong: Profit = 10, so 10 ÷ 50 × 100 = 20%
✓ Right: Profit = 10, so 10 ÷ 40 × 100 = 25%. Divide by the cost price.
Free lesson: Percentages
4. Reverse percentages: taking the percentage of the new value
When you know the value after a change, the new value is not 100%. Divide by the multiplier to get back to the original.
After a 20% increase a price is 360 rials. What was the original price?
✗ Wrong: 20% of 360 = 72, so 360 − 72 = 288 rials
✓ Right: New price = 120% of the original, so original = 360 ÷ 1.2 = 300 rials. Check: 300 × 1.2 = 360.
Free lesson: Percentages
5. Wrong multiplier for growth
A 12% increase uses the multiplier 1.12. Using 0.12 would leave only 12% of the value. For a 12% decrease, use 0.88.
Increase 500 by 12%
✗ Wrong: 500 × 0.12 = 60
✓ Right: 500 × 1.12 = 560
Free lesson: Exponential growth and decay
6. Simple growth instead of compound growth
Compound interest is added to the new total each year, so you multiply, not add the same amount each time. Simple and compound are only equal after one year.
2000 rials is invested at 5% compound interest for 3 years. Find the final value.
✗ Wrong: 5% of 2000 = 100, so 2000 + 3 × 100 = 2300 rials
✓ Right: 2000 × 1.05³ = 2315.25 rials
Free lesson: Exponential growth and decay
Questions students ask
How do I know when to use a reverse percentage?
If you are given the amount after an increase or decrease and asked for the original, it is a reverse percentage. Divide by the multiplier, such as 1.2 for a 20% increase or 0.85 for a 15% decrease.
What is the difference between simple and compound interest?
Simple interest adds the same amount every year, based on the original sum. Compound interest is worked out on the new total each year, so you multiply by the multiplier once for every year.
How can I check a percentage answer quickly?
Estimate. 35% is about a third, 48% is about a half. If your answer is nowhere near, or is bigger than the whole for a percentage under 100%, recheck your method.