Understand the idea.
Make a cost model
A cost model shows what is being bought and how each price is charged. Record the quantity and price of one item. Three notebooks at 4 dollars each cost dollars because the same price is paid three times.
Keep an item price separate from a group cost. Check extra charges carefully: a delivery fee for the whole order is added once, while a charge for each item must be included for every item.
Work through and check
Plan the calculations in the order the situation needs: find each group cost, add all required costs, then compare with the money available. For change, subtract the full cost from the amount paid.
Label the result so it answers the question. If the cost is above the budget, find how much more is needed. Check change by adding it back to the cost; this should give the amount paid.
See the method in action.
Buy groups of items
A class buys 4 books at 7 dollars each and one poster for 6 dollars. It pays with 40 dollars. Find the change.
Worked solution
- 1
Find the cost of four books.
Each of the four books costs 7 dollars.
- 2
Include the poster.
The poster is another required cost, paid once.
- 3
Subtract the full cost from the payment.
Change is the part of the 40-dollar payment not spent.
- 4
Check the change.
The cost and change together account for the full payment.
6 dollars change.
Allow for a once-only fee
A club orders 6 badges at 3 dollars each. Delivery costs 5 dollars for the whole order. The club has 20 dollars. How much more does it need?
Worked solution
- 1
Calculate the badge cost.
There are six badges at 3 dollars each.
- 2
Add delivery once.
The 5-dollar fee covers the whole order rather than each badge.
- 3
Compare the cost with the budget.
The club has less than the full cost, so it needs 3 dollars more.
- 4
Check the amount needed.
Adding the shortage to the budget reaches the total bill.
3 dollars more.
Try it yourself.
Check your understanding
Five pens cost 2 dollars each. One folder costs 4 dollars. What is the total?
Show answer & reasoning
14 dollars
The five pens cost dollars. The folder adds 4 dollars, so dollars altogether.
Keep practising
6 worksheet questions · 12 marks
Read the questions online
- Three rulers cost 4 dollars each. Find the total.1 mark
- Four tickets cost 6 dollars each. A group booking fee is 3 dollars once. Find the full cost.2 marks
- A student pays 30 dollars for 3 packs costing 8 dollars each. Find the change.2 marks
- A class needs 8 pots costing 5 dollars each. It has 36 dollars. How much more is needed?2 marks
- Complete the group costs and the grand total.3 marks
Class project purchases Item Quantity Price each Bags 3 6 dollars Labels 4 2 dollars - Nia buys 2 games at 9 dollars each and pays one delivery fee of 4 dollars. She says the total is 26 dollars. Identify the likely mistake and correct it.2 marks
Read the worked solutions
Equivalent correct methods earn the corresponding marks. Include units where asked.
- Three rulers cost 4 dollars each. Find the total.1 mark
.
12 dollars - Four tickets cost 6 dollars each. A group booking fee is 3 dollars once. Find the full cost.2 marks
.
.
27 dollars - A student pays 30 dollars for 3 packs costing 8 dollars each. Find the change.2 marks
.
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6 dollars - A class needs 8 pots costing 5 dollars each. It has 36 dollars. How much more is needed?2 marks
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4 dollars - Complete the group costs and the grand total.3 marks
Class project purchases Item Quantity Price each Bags 3 6 dollars Labels 4 2 dollars ; .
.
Bags 18 dollars; labels 8 dollars; total 26 dollars. - Nia buys 2 games at 9 dollars each and pays one delivery fee of 4 dollars. She says the total is 26 dollars. Identify the likely mistake and correct it.2 marks
.
The once-only fee gives , not .
She added delivery twice. The total is 22 dollars.
Curriculum reference & lesson scope
Australian Curriculum v9 and SA Curriculum Mathematics 2026
Estimating transactions; multi-step models; responsible purchases; class enterprise and community effects.
Official curriculum source ↗