Topics
5. Addition and subtraction
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AustraliaNSW
Stage 2

5.04 Calculate with money

Lesson
Practice
Lesson

Are you ready?

Do you remember the  value of coins  ?

Examples

Example 1

What is the value of each coin?

a
A 10 cent coin
Worked Solution
Create a strategy

The value of the coin is on the coin. If the coin is silver it is in cents. If the coin is gold it is in dollars.

Apply the idea

Since the coin is silver and there is a 10 on it, the value is 10 cents.

b
A 2 dollar coin
Worked Solution
Create a strategy

The value of the coin is on the coin. If the coin is silver it is in cents. If the coin is gold it is in dollars.

Apply the idea

Since the coin is gold and there is a 2 on it, the value is 2 dollars.

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.

Calculate with money

When we go to the shops, we make purchases of items such as groceries, toys, or clothes. Watch this video to see how we work out the total cost of purchases.

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Examples

Example 2

John wants to buy a toy car for \$22.50. Which of the following collections of money could he use so that he will not get any change?

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

Add the value of each note and coin. 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

All notes and gold coins are for dollars, and the silver coins are for cents. To add the values we can write \$5 as 5.00 and 50 cents as 0.50.

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 \\ & &&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.

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

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

To not get any change we need the amount to equal exactly \$22.50. So the answer is Option B.

Idea summary

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

If we pay for an item with the exact amount of money that the item costs, then we will not get any change back.

Change

If you buy something at a shop, you may need the shopkeeper to give you change. We can count up to work out how much change we should get.

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Examples

Example 3

Which of the following shows the change you would get, if you paid with \$40 and spent \$23.50?

A
An image showing 3 5 dollar notes, a 2 dollar coin and a 50 cent coin.
B
An image showing a 10 dollar note, a 5 dollar note and 3 50 cent coins.
Worked Solution
Create a strategy

Find how much would be the change and compare it to the total amount of money in each option. Change can be found by subtracting the amount spent from the amount paid.

Apply the idea

To find the change we can count up from \$23.50 to \$40.

We would need to count up by another 50 cents to get to \$24. Then if we count up by 6 we get to 30. Then if we count up by 10 we get to 40.

\displaystyle \text{Change}\displaystyle =\displaystyle 50\text{ cents} + \$6 + \$10Add the amounts we counted up by
\displaystyle =\displaystyle \$16.50Add the values

For option A, we can add all the dollars first:

\displaystyle 5+5+5+2\displaystyle =\displaystyle 17Add the dollar values

Then if we also add the 50 cents we get \$17.50.

For option B, we can add all the dollars first:

\displaystyle 10+5\displaystyle =\displaystyle 15Add the dollar values

Now we have three 50 cent coins left. Two of them make \$1 so we can add that to \$15 to get 15+1=\$16

Then if we also add the last 50 cents we get \$16.50. This is equal to the change we found earlier so the answer is Option B.

Idea summary
  • We can add up the cost of each item we are purchasing to find the total.
  • We can calculate the change by finding the difference between the amount we spent and the amount we paid with.

Outcomes

MA2-5NA

uses mental and written strategies for addition and subtraction involving two-, three-, four and five-digit numbers

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