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5.02 Equivalent and simplified ratios

Lesson

Introduction

We previously learned  how to write ratios  that match the information given to us. Writing a ratio can let us compare things mathematically but has only limited use in solving problems. We can build upon this with the use of equivalent ratios and simplified ratios.

Equivalent ratios

Consider a cake recipe that uses 1 cup of milk and 4 cups of flour. What is the ratio of milk to flour used in the cake?

Letting the unit be "the number of cups", we can express the information given as the ratio 1:4.

What if we want to make two cakes? We would need to double the amount of milk and flour we use. This means we will need 2 cups of milk and 8 cups of flour. Now the ratio of milk to flour is 2:8.

But how do we get two different ratios from the same recipe? The secret is that the two ratios actually represent the same proportion of milk to flour. We say that 1:4 and 2:8 are equivalent ratios.

Now consider if we wanted to make enough cakes to use up 4 cups of milk. How many cakes would this make, and how much flour would we need?

Equivalent ratios are useful for when we want to change the value of one quantity but also keep it in the same proportion to another quantity. After calculating how much the value of the first quantity has increased, we can increase the value of the second quantity by the same multiple to preserve the ratio.

We saw in the cake example that increasing both the amount of milk and the amount of flour by the same multiple preserved the ratio. That's because this is the same as having multiple sets of the same ratio.

This image shows 2 squares, 8 circles and 2 cake. Ask your teacher for more information.

Two cakes require twice the ingredients of one cake, but in the same proportion.

And since this is an equivalence relation, we can also say the same for the reverse:

This image shows 1 square, 4 circles and 1 cake. Ask your teacher for more information.

One cake requires half the ingredients of two cakes, but in the same proportion.

Two ratios are equivalent if one of the ratios can be increased or decreased by some multiple to be equal to the other ratio.

Examples

Example 1

The number of students and teachers in a charity race is in the ratio 10:3. If 70 students take part in the race, how many teachers are there?

Worked Solution
Create a strategy

Increase 3 by the same multiple to find the number of teachers.

Apply the idea

We divide 70 by 10 to get the multiple. The multiple is 7.

\displaystyle 10 : 3\displaystyle =\displaystyle 70 : 3 \times 7 Multiply 3 by 7
\displaystyle =\displaystyle 70: 21 Evaluate the multiplication

So, if there are 70 students taking part in the race, there will be 21 teachers.

Example 2

Complete the table of equivalent ratios and use it to answer the following questions.

a
DogstoCats
9:5
18:10
27:
45:
:50
Worked Solution
Create a strategy

Multiply the both sides of ratio by a number to get the equivalent ratio.

Apply the idea
\displaystyle 9:5\displaystyle =\displaystyle 9 \times 3:5 \times 3Multiply by 3
\displaystyle =\displaystyle 27:15Evaluate
\displaystyle =\displaystyle 9\times 5: 5 \times 5Multiply by 5
\displaystyle =\displaystyle 45: 25Evaluate
\displaystyle =\displaystyle 9\times 10: 5 \times 10Multiply by 10
\displaystyle =\displaystyle 90: 50Evaluate

The complete table of equivalent ratios:

DogstoCats
9:5
18:10
27:15
45:25
90:50
b

If there are 270 dogs, how many cats are there expected to be?

A
150
B
30
C
270
D
266
Worked Solution
Create a strategy

Find the multiple number and multiply to the number of cats.

Apply the idea

We can find the multiple by which the number of dogs has increased, by dividing 270 by 9.We get a multiple of 30 and multiply to the number of cats which is 30 \times 5 = 150.

So, if there are 270 dogs, there are 150 cats. The correct option is A.

Idea summary

Two ratios are equivalent if one of the ratios can be increased or decreased by some multiple to be equal to the other ratio.

Simplified ratios

A ratio is a simplified ratio if there is no equivalent ratio with smaller integer values. This is the same as saying that the two integers in the ratio have a greatest common factor of 1.

Since the simplified ratio is the smallest integer valued ratio, this also means that all the ratios equivalent to it are multiples of it. This makes the simplified ratio very useful for solving equivalent ratio questions that don't have very nice numbers.

A ratio is a simplified ratio if there is no equivalent ratio with smaller integer values.

The simplified ratio uses only integers. A ratio that uses fractions or decimals is not yet fully simplified and can be increased or decreased by the appropriate multiple to simplify it.

The application of equivalent and simplified ratios is useful for when we want to keep things in proper proportion while changing their size, or when we want to measure large objects by considering their ratio with smaller objects.

Examples

Example 3

Write 540 cents to \$3.00 as a fully simplified ratio.

Worked Solution
Create a strategy

Convert the dollars into cents then simplify the ratio.

Apply the idea

\$1.00=100\,\text{cents}

\displaystyle 540 : \$ 3.00\displaystyle =\displaystyle 540 : 300 Convert dollars to cents
\displaystyle =\displaystyle \frac{540}{10} : \frac{300}{10} Divide by 10
\displaystyle =\displaystyle 54 : 30 Evaluate the ratio
\displaystyle =\displaystyle \frac{54}{6} : \frac{30}{6} Divide by 6
\displaystyle =\displaystyle 9 : 5 Simplify
Idea summary

A ratio is a simplified ratio if there is no equivalent ratio with smaller integer values.

Outcomes

VCMNA249

Recognise and solve problems involving simple ratios

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