GCSE Maths Ratio and Proportion Ratio

Ratio Problem Solving

# Ratio Problem Solving

Here we will learn about ratio problem solving, including how to set up and solve problems. We will also look at real life ratio problems.

There are also ratio problem solving worksheets based on Edexcel, AQA and OCR exam questions, along with further guidance on where to go next if you’re still stuck.

## What is ratio problem solving?

Ratio problem solving is a collection of word problems that link together aspects of ratio and proportion into more real life questions. This requires you to be able to take key information from a question and use your knowledge of ratios (and other areas of the curriculum) to solve the problem.

A ratio is a relationship between two or more quantities. They are usually written in the form a:b where a and b are two quantities. When problem solving with a ratio, the key facts that you need to know are,

• What is the ratio involved?
• What order are the quantities in the ratio?
• What is the total amount / what is the part of the total amount known?
• What are you trying to calculate?

As with all problem solving, there is not one unique method to solve a problem. However, this does not mean that there aren’t similarities between different problems that we can use to help us find an answer.

The key to any problem solving is being able to draw from prior knowledge and use the correct piece of information to allow you to get to the next step and then the solution.

Let’s look at a couple of methods we can use when given certain pieces of information.

### Use ratio

When solving ratio problems it is very important that you are able to use ratios. This includes being able to use ratio notation.

For example, Charlie and David share some sweets in the ratio of 3:5. This means that for every 3 sweets Charlie gets, David receives 5 sweets.

Charlie and David share 40 sweets, how many sweets do they each get?

We use the ratio to divide 40 sweets into 8 equal parts.

3:5

3 + 5 = 8

40 ÷ 8 = 5

Then we multiply each part of the ratio by 5.

3 x 5:5 x 5 = 15:25

This means that Charlie will get 15 sweets and David will get 25 sweets.

### Dividing ratios

Step-by-step guide: Dividing ratios (coming soon)

You have been given

• the whole amount
• the ratio

• And you want to

• divide a quantity in a ratio
• Step 1: Add the parts of the ratio
together.

Step 2: Divide the quantity by the
sum of the parts.

Step 3: Multiply the share value by each
part in the ratio.
For example

Share £100 in the
ratio 4:1 .

(£80:£20)
You have been given

• part of the total amount
• the ratio

• And you want to find

• the other values in the ratio
• Step 1: Identify which part of the ratio
has been given.

Step 2: Calculate the individual share
value.

Step 3: Multiply the other quantities
in the ratio by the
share value.
For example

A bag of sweets is shared
between boys and girls in
the ratio of 5:6.

same number of sweets. If
there are 15 boys, how many
girls are there?

(18)

### Ratios and fractions (proportion problems)

We also need to consider problems involving fractions. These are usually proportion questions where we are stating the proportion of the total amount as a fraction.

You have been given

• the ratio

• And you want to find

• one part of the ratio as
a fraction of the total
• Step 1: Add the parts of the ratio
for the denominator.

Step 2: State the required part of the
ratio as the numerator.
For example

The ratio of red to green counters
is 3:5. What fraction of
the counters are green?

(\frac{5}{8})
You have been given

• A fraction of an amount

• And you want to find

• The ratio of quantities
• Step 1: Subtract the numerator from
the denominator of the fraction.

Step 2: State the parts of the ratio
in the correct order.
For example

if \frac{9}{10} students are right handed,
write the ratio of right handed
students to left handed students.

(9:1)

### Simplifying and equivalent ratios

Simplifying ratios

You have been given

• the ratio

• And you want to find

• the simplest form
• Step 1: Calculate the highest
common factor of the parts
of the ratio.

Step 2: Divide each part of the
ratio by the highest common
factor.
For example

Simplify the ratio 10:15.

(2:3)

Equivalent ratios

You have been given

• the ratio

• And you want to find

• an equivalent ratio of the
form 1:n
• Step 1: Identify which part of the
ratio is to equal 1.

Step 2: Divide all parts of the
ratio by this value.
For example

Write the ratio 4:15
in the form 1:n.

(1:3.75)
You have been given

• the ratio

• And you want to find

• an equivalent ratio
• Step 1: Multiply all parts of the
ratio by the same amount.
For example

A map uses the scale 1:500.
How many centimetres in real life
is 3cm on the map?

(1:500 = 3:1500, so 1500 cm)

### Units and conversions ratio questions

Units and conversions are usually equivalent ratio problems (see above).

• If £1:\$1.37 and we wanted to convert £10 into dollars, we would multiply both sides of the ratio by 10 to get £10 is equivalent to \$13.70.
• The scale on a map is 1:25,000. I measure 12cm on the map. How far is this in real life, in kilometres? After multiplying both parts of the ratio by 12 you must then convert 12 \times 25000=300000 \ cm to km by dividing the solution by 100 \ 000 to get 3km.

Notice that for all three of these examples, the units are important. For example if we write the mapping example as the ratio 4cm:1km, this means that 4cm on the map is 1km in real life.

Top tip: if you are converting units, always write the units in your ratio.

Usually with ratio problem solving questions, the problems are quite wordy. They can involve missing values, calculating ratios, graphs, equivalent fractions, negative numbers, decimals and percentages.

Highlight the important pieces of information from the question, know what you are trying to find or calculate, and use the steps above to help you start practising how to solve problems involving ratios.

## How to do ratio problem solving

In order to solve problems including ratios:

1. Identify key information within the question.
2. Know what you are trying to calculate.
3. Use prior knowledge to structure a solution.

## Related lessons on ratio

Ratio problem solving is part of our series of lessons to support revision on ratio. You may find it helpful to start with the main ratio lesson for a summary of what to expect, or use the step by step guides below for further detail on individual topics. Other lessons in this series include:

## Ratio problem solving examples

### Example 1: part:part ratio

Within a school, the number of students who have school dinners to packed lunches is 5:7. If 465 students have a school dinner, how many students have a packed lunch?

1. Identify key information within the question.

Within a school, the number of students who have school dinners to packed lunches is \bf{5:7.} If \bf{465} students have a school dinner, how many students have a packed lunch?

Here we can see that the ratio is 5:7 where the first part of the ratio represents school dinners (S) and the second part of the ratio represents packed lunches (P).

We could write this as

Where the letter above each part of the ratio links to the question.

We know that 465 students have school dinner.

2Know what you are trying to calculate.

From the question, we need to calculate the number of students that have a packed lunch, so we can now write a ratio below the ratio 5:7 that shows that we have 465 students who have school dinners, and p students who have a packed lunch.

We need to find the value of p.

3Use prior knowledge to structure a solution.

We are looking for an equivalent ratio to 5:7. So we need to calculate the multiplier. We do this by dividing the known values on the same side of the ratio by each other.

465\div 5 = 93

So the value of p is equal to 7 \times 93=651.

There are 651 students that have a packed lunch.

### Example 2: unit conversions

The table below shows the currency conversions on one day.

Use the table above to convert £520 (GBP) to Euros (EUR).

Identify key information within the question.

Know what you are trying to calculate.

Use prior knowledge to structure a solution.

### Example 3: writing a ratio 1:n

Liquid plant food is sold in concentrated bottles. The instructions on the bottle state that the 500ml of concentrated plant food must be diluted into 2l of water. Express the ratio of plant food to water respectively in the ratio 1:n.

Identify key information within the question.

Know what you are trying to calculate.

Use prior knowledge to structure a solution.

### Example 4: forming and solving an equation

Three siblings, Josh, Kieran and Luke, receive pocket money per week proportional to their age. Kieran is 3 years older than Josh. Luke is twice Josh’s age. If Josh receives £8 pocket money, how much money do the three siblings receive in total?

Identify key information within the question.

Know what you are trying to calculate.

Use prior knowledge to structure a solution.

### Example 5: simplifying ratios

Below is a bar chart showing the results for the colours of counters in a bag.

Express this data as a ratio in its simplest form.

Identify key information within the question.

Know what you are trying to calculate.

Use prior knowledge to structure a solution.

### Example 6: combining two ratios

Glass is made from silica, lime and soda. The ratio of silica to lime is 15:2. The ratio of silica to soda is 5:1. State the ratio of silica:lime:soda.

Identify key information within the question.

Know what you are trying to calculate.

Use prior knowledge to structure a solution.

### Example 7: using bar modelling

India and Beau share some popcorn in the ratio of 5:2. If India has 75g more popcorn than Beau, what was the original quantity?

Identify key information within the question.

Know what you are trying to calculate.

Use prior knowledge to structure a solution.

### Common misconceptions

• Mixing units

Make sure that all the units in the ratio are the same. For example, in example 6 , all the units in the ratio were in millilitres. We did not mix ml and l in the ratio.

• Ratio written in the wrong order

For example the number of dogs to cats is given as the ratio 12:13 but the solution is written as 13:12.

• Ratios and fractions confusion

Take care when writing ratios as fractions and vice-versa. Most ratios we come across are part:part. The ratio here of red:yellow is 1:2. So the fraction which is red is \frac{1}{3} (not \frac{1}{2} ).

• Counting the number of parts in the ratio, not the total number of shares

For example, the ratio 5:4 has 9 shares, and 2 parts. This is because the ratio contains 2 numbers but the sum of these parts (the number of shares) is 5+4=9. You need to find the value per share, so you need to use the 9 shares in your next line of working.

• Ratios of the form \bf{1:n}

The assumption can be incorrectly made that n must be greater than 1 , but n can be any number, including a decimal.

### Practice ratio problem solving questions

1. An online shop sells board games and computer games. The ratio of board games to the total number of games sold in one month is 3:8. What is the ratio of board games to computer games?

3:5

3:11

5:3

8:3

8-3=5 computer games sold for every 3 board games.

2. The volume of gas is directly proportional to the temperature (in degrees Kelvin). A balloon contains 2.75l of gas and has a temperature of 18^{\circ}K. What is the volume of gas if the temperature increases to 45^{\circ}K?

29.75l

6.875l

1.1l

0.91l
\begin{aligned} &2.75l:18^{\circ}K \\\\ &45 \div 18=2.5 \\\\ &2.75 \times 2.5=6.875l \\\\ \end{aligned}

3. The ratio of prime numbers to non-prime numbers from 1-200 is 45:155. Express this as a ratio in the form 1:n.

1:3

1:111

3.4:n

1:3\frac{4}{9}
155\div{45}=3\frac{4}{9}

4. The angles in a triangle are written as the ratio x:2x:3x. Calculate the size of each angle.

30^{\circ}, 60^{\circ}, 90^{\circ}

36^{\circ}, 72^{\circ}, 108^{\circ}

60^{\circ}, 120^{\circ}, 180^{\circ}

10^{\circ}, 20^{\circ}, 30^{\circ}
\begin{aligned} &x+2x+3x=180 \\\\ &6x=180 \\\\ &x=30^{\circ} \\\\ &2x=60^{\circ} \\\\ &3x=90^{\circ} \end{aligned}

5. A clothing company has a sale on tops, dresses and shoes. \frac{1}{3} of sales were for tops, \frac{1}{5} of sales were for dresses, and the rest were for shoes. Write a ratio of tops to dresses to shoes sold in its simplest form.

10:8:6

\frac{1}{3}:\frac{1}{5}:\frac{1}{8}

5:3:7

\frac{5}{15}:\frac{3}{15}:\frac{7}{15}
\begin{aligned} &\frac{1}{3}+\frac{1}{5}=\frac{5+3}{15}=\frac{8}{15} \\\\ &1-\frac{8}{15}=\frac{7}{15}\\\\ &5:3:7 \end{aligned}

6. During one month, the weather was recorded into 3 categories: sunshine, cloud and rain. The ratio of sunshine to cloud was 2:3 and the ratio of cloud to rain was 9:11. State the ratio that compares sunshine:cloud:rain for the month.

2:12:11

6:18:11

2:9:11

6:9:11
\begin{aligned} &3 \times S:C=6:9 \\\\ &6:9:11 \end{aligned}

### Ratio problem solving GCSE questions

1. One mole of water weighs 18 grams and contains 6.02 \times 10^{23} water molecules.

Write this in the form 1gram:n where n represents the number of water molecules in standard form.

(3 marks)

6.02 \div 18=0.334 \ (3dp)

(1)

0.334 \times 10^{23}=3.34 \times 10^{22}

(1)

1:3.34 \times 10^{22}

(1)

2. A plank of wood is sawn into three pieces in the ratio 3:2:5. The first piece is 36cm shorter than the third piece.

Calculate the length of the plank of wood.

(2 marks)

5-3=2 \ parts = 36cm so 1 \ part = 18cm

(1)

18 \times (3+2+5)=180cm

(1)

3. (a) Jenny is x years old. Sally is 4 years older than Jenny. Kim is twice Jenny’s age. Write their ages in a ratio J:S:K.

(b) Sally is 16 years younger than Kim. Calculate the sum of their ages.

(6 marks)

(a)

x+4 or 2x

(1)

x:x+4:2x

(1)

(b)

x+4+16=2x

(1)

x=20

(1)

20+24+40

(1)

=84

(1)

## Learning checklist

You have now learned how to:

• Relate the language of ratios and the associated calculations to the arithmetic of fractions and to linear functions
• Develop their mathematical knowledge, in part through solving problems and evaluating the outcomes, including multi-step problems
• Make and use connections between different parts of mathematics to solve problems

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