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Master the multiplier method and tackle every percentage question type on the IGCSE paper.
Percentages in IGCSE Maths use the multiplier method: a 15% increase uses multiplier 1.15, a 20% decrease uses 0.80. Reverse percentages work backwards from a result to find the original value by dividing by the multiplier. Compound interest applies the multiplier repeatedly: Final = Original x (multiplier)^n where n is the number of periods. These topics carry 8-12 marks across both papers.
Source: Edexcel IGCSE Mathematics (9-1) Specification 4MA1
The multiplier method converts any percentage change into a single multiplication. Instead of calculating the percentage amount and then adding or subtracting, you use one operation. This is faster, less error-prone, and essential for compound problems.
To apply it, simply multiply: New value = Original value x Multiplier. For example, to increase 250 by 12%: 250 x 1.12 = 280.
Reverse percentage questions give you the value after a percentage change and ask you to find the original value. The most common mistake is subtracting the percentage from the given value — this does not give the correct answer.
Example: A shirt costs 156 after a 30% increase. The multiplier is 1.30. Original = 156 / 1.30 = 120. This works because if Original x 1.30 = 156, then Original = 156 / 1.30.
Compound interest means interest is calculated on the original amount plus any interest already earned. It grows faster than simple interest because each period's interest is added to the balance before the next calculation.
Final Amount = P x (1 + r/100)^n
Simple interest uses: Interest = P x r/100 x n. The interest is calculated only on the original principal. Over multiple years, compound interest always gives a higher total because interest earns interest.
Depreciation uses the same compound formula but with a decrease multiplier: Final = P x (1 - r/100)^n. For example, a car depreciating at 15% per year for 3 years: Final = P x 0.85^3.
Percentage change measures how much a value has increased or decreased relative to the original:
Percentage change = ((New - Original) / Original) x 100
If the result is positive, it is an increase. If negative, it is a decrease. The most important detail: you always divide by the original value, not the new value.
Increase 450 by 18%.
Step 1 (Find multiplier): 100 + 18 = 118, so multiplier = 1.18
Step 2 (Multiply): 450 x 1.18 = 531
Answer: 531
After a 25% discount, a jacket costs 135. Find the original price.
Step 1 (Identify change): 25% decrease, so multiplier = 0.75
Step 2 (Reverse — divide): Original = 135 / 0.75 = 180
Answer: The original price was 180
A bank pays 4% compound interest per year. How much will 5,000 be worth after 3 years?
Step 1 (Multiplier): 4% increase = 1.04
Step 2 (Apply formula): Final = 5000 x 1.04^3
Step 3 (Calculate): 5000 x 1.124864 = 5624.32
Answer: 5,624.32
A machine depreciates at 12% per year. After 4 years it is worth 4,500. Find its original value.
Step 1 (Depreciation multiplier): 12% decrease = 0.88
Step 2 (Compound depreciation): Original x 0.88^4 = 4500
Step 3 (Calculate 0.88^4): 0.88^4 = 0.59969536
Step 4 (Reverse): Original = 4500 / 0.59969536 = 7503.81 (2 d.p.)
Answer: 7,503.81 (to 2 decimal places)
Subtracting for reverse percentages: If a value increased by 20% to become 360, students write 360 - 20% of 360 = 288. The correct answer is 360 / 1.20 = 300.
Using simple interest instead of compound: For compound interest questions, you must raise the multiplier to the power n, not multiply by n. 5000 x 1.04 x 3 is wrong; 5000 x 1.04^3 is correct.
Dividing by the new value for percentage change: The denominator must always be the original value, not the new value.
Forgetting to round appropriately: Money answers should be rounded to 2 decimal places unless stated otherwise. Always check what the question asks.
Always write the multiplier first: Before calculating, write down the multiplier clearly (e.g. "multiplier = 1.12"). This earns a method mark even if the arithmetic goes wrong.
Spot the keyword "original": If the question asks for the original amount, it is a reverse percentage. Divide by the multiplier, do not subtract.
Show compound interest year by year if unsure: If you cannot remember the formula, calculate each year separately: Year 1 = P x 1.04, Year 2 = Year 1 x 1.04, and so on. You will still get full marks.
Check with estimation: A 10% increase of 450 is about 495. If your answer is far from that ballpark, recheck your multiplier.
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