Understand the idea.
Compare the whole situation
A sustainable choice considers resources, useful life and waste. An ethical choice also considers effects on people. Price is useful evidence about cost, but does not tell us how an item was made.
Compare options over the same number of uses. A reusable item may cost more to buy once but replace many separate purchases. Check that each option can do the required job for the stated period.
Use evidence to give a reason
Calculate the cost difference and compare any waste information separately. A choice can save money yet produce more wrappers. State which feature supports your decision rather than claiming one number settles everything.
Name important information that the comparison leaves out, such as washing costs or working conditions. A higher purchase price does not prove a product is better, and different budgets can support different reasonable choices.
See the method in action.
Compare twelve uses
A reusable cup costs 12 dollars and lasts for all 12 visits. A single-use cup costs 2 dollars per visit. Compare purchase costs.
Worked solution
- 1
Price the single-use cups for every visit.
A new 2-dollar cup is bought on each of the 12 visits.
- 2
Price the reusable option.
One reusable cup lasts for all the visits stated.
- 3
Compare the purchase totals.
The reusable cup costs 12 dollars less over these 12 visits.
- 4
State the limit of this comparison.
These purchase totals do not include washing or travel costs, so they do not describe every cost.
The reusable cup costs 12 dollars less for the 12 visits.
Balance cost and waste
Option A costs 18 dollars and produces 12 wrappers. Option B costs 21 dollars and produces 3 wrappers. The class has 25 dollars. Compare them.
Worked solution
- 1
Check both options fit the budget.
Neither option exceeds the 25 dollars available.
- 2
Find the extra cost of B.
B uses 3 dollars more of the budget than A.
- 3
Find the reduction in wrappers.
B produces nine fewer wrappers for the stated purchase.
- 4
Explain a choice using a priority.
A is supported by lower cost; B is supported by fewer wrappers. The preferred priority must be stated.
B costs 3 dollars more and produces 9 fewer wrappers; either choice needs a stated reason.
Try it yourself.
Check your understanding
Repairing a pencil case costs 4 dollars. A new one costs 11 dollars. Both will work for the same stated time. Find the cost difference.
Show answer & reasoning
7 dollars
The cost difference is dollars, so repair costs 7 dollars less over the same stated working time. Other comparisons need further evidence.
Keep practising
6 worksheet questions · 15 marks
Read the questions online
- A reusable bag costs 6 dollars and lasts for six trips. Single-use bags cost 2 dollars per trip. Compare purchase totals for six trips.2 marks
- A chair repair costs 15 dollars. A replacement costs 42 dollars. Find the cost difference.2 marks
- A refill costs 5 dollars. A new bottle costs 8 dollars. Compare 4 refills with 4 new bottles, assuming a refill bottle is already owned.3 marks
- For a 30-dollar budget, compare the options and give a reason for choosing either.3 marks
Suitable class supplies Option Cost (dollars) Packaging pieces A 24 10 B 27 2 - A lunch box costs 10 dollars and lasts for 20 lunches. Disposable containers cost 1 dollar per lunch. Compare purchase costs for 20 lunches.2 marks
- A shop says a 16-dollar item is more sustainable than a 12-dollar item because it costs more. Is this enough evidence? Give one useful question.3 marks
Read the worked solutions
Equivalent correct methods earn the corresponding marks. Include units where asked.
- A reusable bag costs 6 dollars and lasts for six trips. Single-use bags cost 2 dollars per trip. Compare purchase totals for six trips.2 marks
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Reusable: 6 dollars; single-use: 12 dollars. The reusable bag costs 6 dollars less. - A chair repair costs 15 dollars. A replacement costs 42 dollars. Find the cost difference.2 marks
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27 dollars - A refill costs 5 dollars. A new bottle costs 8 dollars. Compare 4 refills with 4 new bottles, assuming a refill bottle is already owned.3 marks
Refills: .
New bottles: .
.
The refills cost 12 dollars less. - For a 30-dollar budget, compare the options and give a reason for choosing either.3 marks
Suitable class supplies Option Cost (dollars) Packaging pieces A 24 10 B 27 2 Cost difference: .
Packaging difference: .
Explain which information matters to the choice.
Both fit. A costs 3 dollars less; B makes 8 fewer packaging pieces. Either choice is valid with a stated priority. - A lunch box costs 10 dollars and lasts for 20 lunches. Disposable containers cost 1 dollar per lunch. Compare purchase costs for 20 lunches.2 marks
.
.
Assume the lunch box works for all 20 lunches.
Lunch box: 10 dollars; disposable: 20 dollars. Difference: 10 dollars. - A shop says a 16-dollar item is more sustainable than a 12-dollar item because it costs more. Is this enough evidence? Give one useful question.3 marks
Price alone does not measure sustainability.
For example: How long will each item last?
Accept another relevant evidence-based question.
No. Ask about useful life, repair, materials or packaging.
Curriculum reference & lesson scope
Australian Curriculum v9 and SA Curriculum Mathematics 2026
Practical models; immediate/future financial plans; consumer goals; ethical/sustainable choices.
Official curriculum source ↗