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Problem Solving with Ratios (Cambridge (CIE) IGCSE International Maths)

Revision Note

Naomi C

Written by: Naomi C

Reviewed by: Dan Finlay

Cambridge (CIE) International Maths: CoreCambridge (CIE) Maths: CoreCambridge (CIE) International Maths: ExtendedCambridge (CIE) Maths: Extended

Ratio & Proportion

What type of ratio problems could I be asked to solve?

  • Simple ratio problems are discussed in earlier revision notes, including

    • Writing ratios

    • The link between ratios and fractions 

    • Equivalent ratios

    • Simplifying ratios

    • Sharing an amount in a given ratio 

  • Further problems involving ratio include

    • Ratios where you are given the difference between the two parts

      • E.g. Kerry is given $30 more than Kacey who is given $50

    • Ratios where one quantity is given and you have to find the other quantity

      • E.g. Kerry and Kacey are sharing money in the ratio 8 : 5, Kacey gets $50

    • Situations where you are given two separate (two-part) ratios but can combine them in to one (three-part) ratio

      • E.g. Kerry and Kacey are sharing money in the ratio 8 : 5 whilst Kacey is also sharing money with Kylie in the ratio 1 : 2

How do I solve a ratio problem when given the difference between two parts?

  • Find the difference in the number of parts between the two quantities in the ratio

  • Compare the difference in the number of parts with the difference between the actual numbers 

  • Simplify to find out the value of one part

  • Multiply the value of one part by the number of parts for each quantity in the ratio 

  • Multiply the value of one part by the total number of parts to find the total amount

Given one quantity of a ratio, how can I find the other quantity?

  • Compare the given quantity with the relevant number of parts in the ratio

  • Simplify to find the value of one part

  • Multiply the value of one part by the number of parts in the remaining quantity in the ratio

  • Multiply the value of one part by the total number of parts to find the total amount

How do I combine two ratios to make a three-part ratio?

  • Identify the link between the two different ratios 

  • Find equivalent ratios for both original ratios, where the value of the link is the same

  • Join the two, two-part ratios into a three-part ratio

Worked Example

(a) The ratio of cabbage leaves eaten by two rabbits, Alfred and Bob, is 8 : 4 respectively. It is known that Alfred eats 12 more cabbage leaves than Bob for a particular period of time. Find the total number of cabbage leaves eaten by the rabbits and the number that each rabbit eats individually.

The difference in the number of parts is

8 - 4 = 4 parts

This means that

4 parts = 12 cabbage leaves

Dividing both by 4

1 part = 3 cabbage leaves

Find the total number of parts

8 + 4 = 12 parts

Find the total number of cabbage leaves

12 × 3 = 36

36 cabbage leaves in total

Find the number eaten by Alfred

8 × 3 = 24

24 cabbage leaves

Find the number eaten by Bob

4 × 3 = 12

12 cabbage leaves

(b) A particular shade of pink paint is made using 3 parts red paint, to two parts white paint.

Mark already has 36 litres of red paint, but no white paint. Calculate the volume of white paint that Mark needs to purchase in order to use all of his red paint, and calculate the total amount of pink paint this will produce.

The ratio of red to white is

3 : 2

Mark already has 36 litres of red, so

36 litres = 3 parts

Divide both sides by 3

12 litres = 1 part

The ratio was 3 : 2
Find the volume of white paint, 2 parts

2 × 12 = 24

24 litres of white paint

In total there are 5 parts, so the total volume of paint will be

5 × 12 = 60

60 litres in total

 

(c) In Jamie’s sock drawer the ratio of black socks to striped socks is 5 : 2 respectively. The ratio of striped socks to white socks in the drawer is 6 : 7 respectively.

Calculate the percentage of socks in the drawer that are black.

Write down the ratios

B : S = 5 : 2
S : W = 6 : 7

S features in both ratios, so we can use it as a link
Multiply the B : S ratio by 3 to find an equivalent ratio
Both ratios are now comparing to 6 striped socks

B : S = 15 : 6
S : W = 6 : 7

Link them together

B : S : W = 15 : 6 : 7

Find the total number of parts

15 + 6 + 7 = 28

This means 15 out of 28 socks are black
Find 15 out of 28 as a decimal by completing the division

15 over 28 equals 0.535 space 714 space 285 space...

Convert to a percentage
Multiply by 100 and round to 3 significant figures

53.6 % of the socks are black

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    Author
    Reviewer
    Naomi C

    Author: Naomi C

    Expertise: Maths

    Naomi graduated from Durham University in 2007 with a Masters degree in Civil Engineering. She has taught Mathematics in the UK, Malaysia and Switzerland covering GCSE, IGCSE, A-Level and IB. She particularly enjoys applying Mathematics to real life and endeavours to bring creativity to the content she creates.

    Dan Finlay

    Author: Dan Finlay

    Expertise: Maths Lead

    Dan graduated from the University of Oxford with a First class degree in mathematics. As well as teaching maths for over 8 years, Dan has marked a range of exams for Edexcel, tutored students and taught A Level Accounting. Dan has a keen interest in statistics and probability and their real-life applications.