Topics
6. Decimals
topic badge
Australia
Year 4

6.06 Purchases and change

Lesson
Practice
Lesson

Are you ready?

We need to remember how to  calculate the total  amount of money that we have. Let's try this problem.

Examples

Example 1

Which of the following shows a total of \$22.50?

A
An image showing a 20 dollar note, a 2 dollar coin and a 50 cent coin.
B
An image showing a 20 dollar note, a 5 dollar note and two 1 dollar coins
Worked Solution
Create a strategy

Add the value of each note and coin using the vertical algorithm. Use this table of values to help you.

A table showing the value of 2 dollar, 1 dollar, 50 cent, 20 cent, 10 cent and 5 scent coins.
Apply the idea

For Option A, write the addition of values in a vertical algorithm and add the digits in each column.

\begin{array}{c} & &2&0&.&0&0 \\ & &&2&.&0&0 \\ &+&&0&.&5&0 \\ \hline & &2&2 &. &5&0 \\ \hline \end{array}

The total amount of notes and coins in Option A is \$22.50.

For Option B, write the addition of values in a vertical algorithm and add the digits in each column.

\begin{array}{c} & &2&0&.&0&0 \\ & &&5&.&0&0 \\ &&&1&.&0&0 \\ &+&&1&.&0&0 \\ \hline & &2&7 &. &0&0 \\ \hline \end{array}

The total amount of notes and coins in Option B is \$27.00.

So the correct answer is option A.

Idea summary

The value of a coin is written on the coin. Silver coins are for cents, and gold coins are for dollars.

All notes have dollar values. The value is written on the note.

To add money we can write the values as decimals and add them using a vertical algorithm.

Money and change

Let's learn more about calculating change.

Loading video...

Examples

Example 2

How much change would you receive from \$10 if you spent \$7.50?

Worked Solution
Create a strategy

Use a number line and count back from 10 by 7.50.

Apply the idea

Plot 10 on the number line:

012345678910

Count back by 7 to get to 3:

012345678910

Count back by 0.5 to get to 2.5:

012345678910
\displaystyle \text{Change}\displaystyle =\displaystyle 10-7.50
\displaystyle =\displaystyle \$2.50
Idea summary

We can either count up or count back to find the difference between the amount of money we start with and the amount of money that we spent.

Outcomes

AC9M4N07

choose and use estimation and rounding to check and explain the reasonableness of calculations including the results of financial transactions

AC9M4N08

use mathematical modelling to solve practical problems involving additive and multiplicative situations including financial contexts; formulate the problems using number sentences and choose efficient calculation strategies, using digital tools where appropriate; interpret and communicate solutions in terms of the situation

What is Mathspace

About Mathspace