The idea, step by step
Ask what the purchase must do
A useful comparison begins with the need. How many items are required? What is the budget? Is there a feature that matters, such as being washable or able to be reused? A cheap pack is not a good solution if it does not supply enough.
Separate a need from an extra preference. If the class needs twelve markers, the count matters. A favourite colour may be a preference. You can choose a preference when it still meets the need and budget.
Compare complete options
Compare the full cost of enough packs, not just the displayed pack price. Two packs of six markers may be enough, while one pack of four is not. Make a small table so the same quantities are being compared.
There may be several sensible choices. State the criterion you used: lowest total cost, less waste, a useful feature or a longer usable life. Do not claim a product lasts longer unless that information is given.
A common trap
The pack with the lowest price must be the best buy.
Instead: First check how many packs are needed. Compare the full costs of enough items and any stated useful features.
See the thinking
Buy enough markers
A class needs 12 markers. Pack A has 6 markers for 8 dollars. Pack B has 4 for 6 dollars. Which supplies 12 for less money?
- A needs 2 packs: 2 × 6 = 12 markers, costing 2 × 8 = 16 dollars.
- B needs 3 packs: 3 × 4 = 12 markers, costing 3 × 6 = 18 dollars.
- Both meet the count. A costs 2 dollars less.
Use stated features
A reusable bottle costs 8 dollars. A disposable drink bottle costs 3 dollars each day. A learner needs a bottle on four days and can refill the reusable bottle for free. Which costs less for these days?
- Four disposable bottles cost 4 × 3 = 12 dollars.
- The reusable bottle costs 8 dollars once.
- 8 is 4 dollars less than 12. It also uses one bottle instead of four.
Check your understanding
You need 10 labels. A pack of 4 costs 3 dollars. Is buying two packs enough?
Show answer
No.
Two packs give 8 labels, which is fewer than 10. Three packs give 12 labels and cost 9 dollars.
Your turn
6 questions · 12 marks · Work at your own pace.
Read the questions online
- You need 9 cups. Packs contain 3 cups and cost 4 dollars. How many packs and what total cost?2 marks
- Compare enough stickers for 12 children: A has 6 for 5 dollars; B has 4 for 4 dollars. Find the cheaper option.2 marks
- A class has a 20-dollar budget for 12 markers. Pack A has 6 for 8 dollars. Can the class afford enough? State the money left.2 marks
- A pen costing 5 dollars can be used for the whole term. Three short-life pens costing 2 dollars each are needed for the same term. Which has the lower stated term cost?2 marks
- A bag costs 9 dollars. A second bag costs 12 dollars and has a waterproof pocket. Your budget is 12 dollars and both carry everything needed. Give a justified choice.2 marks
- A purchase costs 18 dollars. Delivery is 4 dollars. A learner has 20 dollars and says it is affordable because 18 is below 20. Explain.2 marks
Read the worked solutions
Equivalent correct methods earn the corresponding marks. Include units where asked.
- You need 9 cups. Packs contain 3 cups and cost 4 dollars. How many packs and what total cost?2 marks
9 ÷ 3 = 3 packs.
3 × 4 = 12.
3 packs; 12 dollars - Compare enough stickers for 12 children: A has 6 for 5 dollars; B has 4 for 4 dollars. Find the cheaper option.2 marks
A needs 2 packs; 2 × 5 = 10.
B needs 3 packs; 3 × 4 = 12.
A: 10 dollars, compared with B: 12 dollars. - A class has a 20-dollar budget for 12 markers. Pack A has 6 for 8 dollars. Can the class afford enough? State the money left.2 marks
Two packs provide 12 markers and cost 16 dollars.
20 − 16 = 4.
Yes; 4 dollars remains. - A pen costing 5 dollars can be used for the whole term. Three short-life pens costing 2 dollars each are needed for the same term. Which has the lower stated term cost?2 marks
Three short-life pens cost 3 × 2 = 6 dollars.
6 − 5 = 1.
The 5-dollar pen, by 1 dollar. - A bag costs 9 dollars. A second bag costs 12 dollars and has a waterproof pocket. Your budget is 12 dollars and both carry everything needed. Give a justified choice.2 marks
Both meet the need and budget.
State a given feature or a cost saving; do not invent extra product claims.
Either: choose 9 dollars to save 3 dollars, or choose 12 dollars if the waterproof pocket matters. - A purchase costs 18 dollars. Delivery is 4 dollars. A learner has 20 dollars and says it is affordable because 18 is below 20. Explain.2 marks
18 + 4 = 22.
All required costs must be included.
It is not affordable with delivery; the total is 22 dollars, 2 dollars too much.
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 ↗