Ethical/sustainable choices | Year 5

01 / LEARN

The idea, step by step

Compare the whole situation

A sustainable choice may use less material, repair an item or reuse something already available. An ethical choice also considers effects on people. Mathematics can compare costs and quantities, but cannot reveal every fact about how an item was made.

Use the information supplied. A reusable container may cost more at first but replace several single-use containers. The comparison depends on the number of uses and whether each item is suitable.

Use evidence to give a reason

Set a fair number of uses or time period. Calculate both totals for that period and compare the information about waste. If a useful fact is missing, explain what else you would need to know.

Do not call something better just because it costs more, or assume every family can make the same choice. Explain a decision using costs, purpose and the circumstances. Different answers may be reasonable.

A common trap

Use price alone to claim that a product is more sustainable.

Instead: Price does not show useful life, waste or how an item was made. Use relevant evidence and state missing information.

02 / WORKED EXAMPLES

See the thinking

Example 1

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.

Single-use cups: $12 \times 2 = 24$ dollars.
Reusable cup: 12 dollars once.
$24 - 12 = 12$ dollars less.
This comparison leaves out washing and travel costs.

The reusable cup costs 12 dollars less for the 12 visits.

Example 2

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.

Both costs fit the budget.
B costs $21 - 18 = 3$ dollars more.
B produces $12 - 3 = 9$ fewer wrappers.
A lower-cost priority could support A; a fewer-wrappers priority could support B.

B costs 3 dollars more and produces 9 fewer wrappers; either choice needs a stated reason.

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

7 dollars

$11 - 4 = 7$ dollars. Further comparisons would need more information.

03 / WORKSHEET

Your turn

6 questions · 15 marks · Work at your own pace.

Worksheet PDF Worked solutions

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

Suitable class supplies
Option	Cost (dollars)	Packaging pieces
A	24	10
B	27	2

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
$6 \times 2 = 12$ .
$12 - 6 = 6$ .
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
$42 - 15 = 27$ .
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: $4 \times 5 = 20$ .
New bottles: $4 \times 8 = 32$ .
$32 - 20 = 12$ .
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: $27 - 24 = 3$ .
Packaging difference: $10 - 2 = 8$ .
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
$20 \times 1 = 20$ .
$20 - 10 = 10$ .
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.

Suitable class supplies
Option	Cost (dollars)	Packaging pieces
A	24	10
B	27	2

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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 ↗

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