Understand the idea.
Ask what the purchase must do
Start a purchase comparison with what is needed: the number of items, the money available and any required features. A pack cannot meet the need just because its price is small. It must provide enough usable items.
A preference, such as a favourite colour, is different from a requirement. First find options that meet the need and budget. Then use the given preferences or features to explain a choice among those options.
Compare complete options
Compare the total cost of enough items from each option. A pack of four and a pack of six supply different quantities, so comparing their pack prices alone can be misleading. Find how many complete packs are required.
State the reason for your choice: lower total cost, fewer unused items or a stated useful feature. Only use information supplied in the question. A more expensive item is not automatically stronger, longer-lasting or better.
See the method in action.
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?
Worked solution
- 1
Find the number of A packs needed.
Two complete packs supply exactly the 12 markers required.
- 2
Calculate the cost of A.
Each of those two packs costs 8 dollars.
- 3
Find the cost of enough B packs.
Three B packs are required to supply the same 12 markers.
- 4
Compare like quantities.
Both supply 12 markers, but two A packs cost 2 dollars less.
Two packs of A cost 16 dollars and are cheaper.
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?
Worked solution
- 1
Calculate four daily purchases.
The disposable option requires a new 3-dollar bottle on each of four days.
- 2
Find the reusable option cost.
One reusable bottle is bought once and the question gives free refills.
- 3
Compare the four-day costs.
The reusable bottle supplies the stated need for 4 dollars less.
The reusable bottle costs 4 dollars less under the stated conditions.
Try it yourself.
Check your understanding
You need 10 labels. A pack of 4 costs 3 dollars. Is buying two packs enough?
Show answer & reasoning
No.
Two packs provide labels, fewer than the 10 needed. Three packs provide labels and cost dollars.
Keep practising
6 worksheet questions · 12 marks
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
packs.
.
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; .
B needs 3 packs; .
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.
.
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 dollars.
.
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
.
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 ↗