Percentages

Edexcel International GCSE Mathematics A· 1.6· 45 分钟阅读

1. Percentage Fundamentals and Conversions★☆☆☆☆⏱ 8 min

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A percentage means 'number of parts per 100', and is a common way to represent proportions, changes and comparisons in everyday life and exam problems.

📘 定义

Percentage

A ratio that represents a number of parts per 100, denoted with the % symbol

例:

25% means 25 parts out of 100, equal to $\frac{25}{100}$ or 0.25

To convert a percentage to a decimal: divide by 100
To convert a decimal to a percentage: multiply by 100
To convert a percentage to a fraction: write the value over 100 and simplify fully

📐 例题

Convert 35% to a simplified fraction and a decimal, and convert 0.04 to a percentage.

1
Write 35% as a fraction over 100 and simplify:
$35\% = \frac{35}{100} = \frac{7}{20}$
2
Divide 35 by 100 to get the decimal equivalent:
$35 \div 100 = 0.35$
3
Multiply 0.04 by 100 to get the percentage:
$0.04 \times 100 = 4\%$

Exam tip:

Always simplify fractions fully when asked to express a percentage as a fraction to gain full marks.

2. Percentage of Amounts and Simple Change★★☆☆☆⏱ 10 min

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The multiplier method is the fastest, most accurate way to calculate percentages of amounts and percentage increases or decreases. A multiplier is the decimal equivalent of the percentage you are applying.

📘 定义

Percentage Multiplier

Decimal value used to calculate percentage operations in one step:
- Percentage of an amount: multiplier = $r /100$ for r%
- Increase by r%: multiplier = $1 + r /100$
- Decrease by r%: multiplier = $1 - r /100$

📐 例题

Calculate 18% of £240, then find the new price of a £240 jacket after an 18% price increase.

1
Calculate the 18% multiplier:
$18/100 = 0.18$
2
Find 18% of £240:
$0.18 \times 240 = £43.20$
3
Calculate the increase multiplier:
$1 + 0.18 = 1.18$
4
Find the new jacket price:
$240 \times 1.18 = £283.20$

3. Reverse Percentages★★★☆☆⏱ 10 min

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Reverse percentage questions ask you to find the original value of a quantity before a known percentage change was applied. The key rule is to divide by the change multiplier, do not add or subtract the percentage from the final value.

📐 例题

A shirt is on sale for £42 after a 30% discount. Find the original price of the shirt.

1
Calculate the discount multiplier for 30% off:
$1 - 30/100 = 0.7$
2
Set up the relationship between original and sale price:
$Original \times 0.7 = 42$
3
Rearrange to find the original price:
$Original = 42 \div 0.7 = £60$

Exam tip:

Always check your reverse percentage answer by applying the given percentage change to your result to confirm you get the stated final value.

4. Compound Interest and Depreciation★★★☆☆⏱ 10 min

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Compound interest and depreciation use repeated multipliers to calculate value change over multiple time periods. For n periods, you raise the annual multiplier to the power of n. No formula is provided for this topic, so you must recall this method.

Compound interest: interest is added to the principal each period, so interest is earned on previous interest
Depreciation: the value of an asset decreases by a fixed percentage each period

📐 例题

Calculate the value of a £1500 investment after 3 years with 4% annual compound interest. Give your answer to the nearest penny.

1
Calculate the annual interest multiplier:
$1 + 4/100 = 1.04$
2
Calculate the total multiplier for 3 years:
$1.04^3 = 1.124864$
3
Calculate the final value and round to 2 decimal places:
$1500 \times 1.124864 = £1687.30$

5. Repeated Percentage Change (Higher Only)★★★★☆Higher 专属⏱ 7 min

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When multiple percentage changes are applied sequentially, the total change is found by multiplying all individual change multipliers together. You cannot add or subtract the percentage values directly, as each change applies to a different base value.

📐 例题

A shop increases all prices by 30%, then runs a 20% off sale for all items. What is the total percentage change in price?

1
Calculate the 30% increase multiplier:
$1 + 30/100 = 1.3$
2
Calculate the 20% decrease multiplier:
$1 - 20/100 = 0.8$
3
Find the total multiplier by multiplying the two values:
$1.3 \times 0.8 = 1.04$
4
Convert the multiplier to percentage change: 1.04 = 104%, so total change is a 4% increase.

Exam tip:

The order of percentage changes does not affect the final total multiplier, so you can multiply them in any order to cross-check your answer.

6. 常见陷阱

错误做法:

Converting percentage to decimal by multiplying by 100 instead of dividing

原因:

Mixes up conversion directions, leading to values 100x larger than correct

正确做法:

Divide percentages by 100 to get decimal multipliers, multiply decimals by 100 to get percentages

错误做法:

Adding the discount percentage to the sale price for reverse percentage questions, e.g. £42 + 30% of 42 = £54.60 for the earlier shirt example

原因:

The 30% discount applies to the original price, not the sale price

正确做法:

Divide the final value by the percentage change multiplier to get the original value

错误做法:

Rounding intermediate multiplier powers for compound interest, e.g. rounding $1.0 4^{3}$ to 1.12 early

原因:

Rounding errors accumulate, leading to incorrect final answers

正确做法:

Keep all unrounded intermediate values in your calculator, only round the final answer

错误做法:

Calculating simple interest instead of compound interest by multiplying by n instead of raising to power n

原因:

Confuses simple and compound growth, leading to understated final values

正确做法:

Raise the annual multiplier to the power of the number of periods for compound growth or decay

错误做法:

Adding sequential percentage changes directly, e.g. 30% increase then 20% decrease = 10% total increase

原因:

Each percentage change applies to a different base value, so they cannot be added directly

正确做法:

Multiply all individual percentage change multipliers together to get the total change multiplier

7. 速查表

Task	Method	Example
Convert % to decimal	Divide by 100	12% = 0.12
Convert decimal to %	Multiply by 100	0.65 = 65%
% increase by r%	× $(1 + r /100)$	10% increase: × 1.1
% decrease by r%	× $(1 - r /100)$	15% decrease: × 0.85
Reverse % (find original)	Divide by change multiplier	Sale price £56 after 20% off: $56 \div 0.8 = £70$
Compound interest n years	× $(m u l t i pl i er)^{n}$	£1000 at 5% for 2 years: $1000 \times 1.0 5^{2} = £1102.50$
Repeated % change	Multiply all multipliers	20% increase then 10% decrease: $1.2 \times 0.9 = 1.08$ (8% increase)

8. 常见问题

Do I get the compound interest formula on the exam formula sheet?

No, you must recall how to use repeated multipliers for compound interest and depreciation. No formula for this topic is provided in the Edexcel IGCSE Maths A formula sheet.

How do I round money answers for percentage problems?

Always round final money answers to 2 decimal places (nearest penny/cent) unless the question explicitly specifies otherwise. Keep intermediate values unrounded in your calculator to avoid rounding errors.

Can I add sequential percentage changes together to get the total change?

No, each percentage change applies to a different base value, so you cannot add them directly. Multiply the individual multipliers together to find the total change multiplier.

下一步

Now that you have mastered percentage calculations for Edexcel IGCSE Maths A, you can apply these skills to more advanced topics in the numbers unit, including ratio and proportion, and practical problem-solving questions involving money and finance. Percentages are also frequently tested alongside statistical topics such as data interpretation and probability, where you will need to calculate percentage changes from graphs and tables. Make sure to practice both foundation and higher tier past paper questions to solidify your understanding, paying close attention to rounding requirements for money answers and showing all multiplier steps to gain full marks.