Ratio and Proportion
Edexcel International GCSE Mathematics A· 1.7 (2016 Specification)· 18 min read
1. Ratio Notation & Simplification★★☆☆☆⏱ 4 min
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Simplified Ratio
A ratio written with integers that have a highest common factor (HCF) of 1, or in the required 1:n format for unit comparison.
Example:
12:18 simplifies to 2:3 (integer simplest form) or 1:1.5 (1:n form).
Ratios compare relative sizes of 2+ quantities, written with colons between parts. Always align units across all parts before simplifying, then divide every part by their shared HCF to get integer simplest form. If asked for 1:n format, divide all parts by the first value to set the first part equal to 1.
Simplify the ratio 240g : 1.2kg, then express it in the form 1:n.
- 1
- Align units: convert 1.2kg to 1200g, so the ratio is 240:1200
- 2
- Calculate HCF of 240 and 1200 = 240, divide both parts by 240 to get 1:5
- 3
- The first part is already 1, so the 1:n form is 1:5
Exam tip:
Always check units first when simplifying ratios; mismatched units are the top error for this question type.
2. Sharing Quantities in a Given Ratio★★★☆☆⏱ 5 min
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To split a quantity into parts matching a given ratio, first calculate the total number of parts in the ratio, find the value of 1 single part, then multiply by the number of parts for each share. Always sum your final shares to confirm they match the original total.
Share £416 between three people in the ratio 4:3:1.
- 1
- Calculate total parts: 4 + 3 + 1 = 8 parts
- 2
- Value of 1 part = £416 ÷ 8 = £52
- 3
- Calculate each share: 4 × £52 = £208, 3 × £52 = £156, 1 × £52 = £52
- 4
- Verify total: £208 + £156 + £52 = £416, matching the original amount
Exam tip:
Spend 10 seconds adding up your final shares to catch calculation errors; this check will save you easy lost marks.
3. Direct Proportion Using Scaling★★★☆☆⏱ 4 min
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Unitary Method for Proportion
A numerical method to solve direct proportion problems by first calculating the value of 1 unit, then scaling up or down to find the required quantity.
Example:
If 3 pens cost £1.20, 1 pen costs £0.40, so 7 pens cost 7 × £0.40 = £2.80.
Two quantities are directly proportional if when one increases, the other increases by the same factor. For S1 questions, use only unitary or factor scaling methods, not algebraic proportionality constants (assessed in later topics).
The mass of 12 identical wooden blocks is 3.6kg. Find the mass of 20 such blocks.
- 1
- Find mass of 1 block: 3.6kg ÷ 12 = 0.3kg per block
- 2
- Scale to 20 blocks: 20 × 0.3kg = 6kg
- 3
- Verify with factor scaling: 20/12 = 5/3, so 3.6kg × 5/3 = 6kg, confirming the result
Exam tip:
Use whichever method is faster for the numbers given: unitary method is easiest for small values, factor scaling works well for clean multiples.
4. Ratio in Maps & Scale Diagrams★★★☆☆⏱ 5 min
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Map scales are written as ratios, where the first number is the distance on the map, and the second is the actual distance in the same unit. Memorize that 1km = 100,000cm to simplify unit conversion for scale questions.
A map has a scale of 1:25,000. The distance between two villages on the map is 8cm. Calculate the actual distance between the villages in kilometres.
- 1
- 1cm on map = 25,000cm actual distance
- 2
- 8cm × 25,000 = 200,000cm total actual distance
- 3
- Convert cm to km: 200,000 ÷ 100,000 = 2km
Exam tip:
The first value in any scale ratio always refers to the diagram/map measurement, never the real-world measurement. This is a common mix-up in exams.
5. Common Pitfalls
Wrong move:
Simplifying ratios with mismatched units (e.g. 200m : 1km simplified to 200:1)
Why:
Units must be identical to compare relative sizes of quantities
Correct move:
Convert all quantities to the same unit before simplifying ratios
Wrong move:
Dividing total quantity by the first ratio part instead of total parts when sharing
Why:
The ratio represents parts of the whole, so you need the sum of all parts to find the value of 1 unit
Correct move:
Add all parts of the ratio to get total parts, then divide the total quantity by this sum
Wrong move:
Mixing up map scale parts (e.g. assuming 25,000cm map = 1cm actual)
Why:
Map scales are always written as [map distance]:[actual distance]
Correct move:
Remember the first number in any scale ratio refers to the diagram or map measurement
Wrong move:
Leaving ratios as decimals when asked for simplest integer form
Why:
Standard simplest form requires integer values with HCF 1, unless 1:n format is explicitly requested
Correct move:
Check question instructions: use integers for standard simplified ratios, decimals only for 1:n form
Wrong move:
Using algebraic proportionality constant k for S1 proportion questions
Why:
This topic assesses numerical scaling only, and algebraic methods introduce unnecessary error risk
Correct move:
Use unitary or factor scaling methods for all S1 ratio and proportion questions
6. Quick Reference Cheatsheet
Task | Key Steps | Quick Example |
|---|---|---|
Simplify ratio |
| 240g:1.2kg = 240:1200 = 1:5 |
Share quantity in ratio |
| Share £416 in 4:3:1: 8 parts = £52 each, shares £208, £156, £52 |
Solve direct proportion |
| 12 blocks = 3.6kg, 20 blocks = 6kg |
Calculate map scale distance |
| 1:25000 scale, 8cm map = 2km actual |
7. Frequently Asked
Do I need to use the constant k for proportion problems in this topic?
No, for Section S1 proportion questions, you only need to use unitary scaling or ratio methods. The algebraic constant of proportionality k is assessed separately in Section 2.5 (Higher tier only).
What counts as a simplified ratio?
Unless explicitly asked for the 1:n format, a simplified integer ratio has a highest common factor (HCF) of 1 between all parts, with no decimals or fractions.
Going deeper
What's Next
Now that you have mastered basic ratio and proportion for Section S1, you can move on to related number topics including percentages and compound interest, before progressing to algebraic proportion (Section 2.5) for Higher tier content. Practicing past paper ratio questions will help you build speed and avoid common unit conversion errors, which are frequently tested in both calculator and non-calculator papers. These ratio skills also appear frequently in multi-part geometry and statistics questions, so mastering them now will boost your performance across the full exam syllabus.