Sustainable Budgeting

Overview

In lesson 4 of the 10-lesson unit Design a Sustainable Community, students develop a budget for a sustainable community space. Building on prior work with rational numbers, ratios and financial contexts, students calculate percentage discounts, mark-ups, GST and contingency allowances, compare suppliers, and justify whether their model remains within a stated budget.

Learning intentions

• WALT create and analyse a sustainable-space budget using percentages.
• WALT calculate discounts, mark-ups, GST and contingency allowances.
• WALT compare supplier options using tables or a spreadsheet.
• WALT communicate and justify a financially appropriate recommendation.

Success criteria

• I can calculate percentage changes accurately and explain whether they increase or decrease a cost.
• I can organise costs in a table or spreadsheet, including totals and allowances.
• I can compare supplier options using evidence.
• I can remain within the stated budget and explain whether my model is realistic.

Curriculum links

• Number — mathematical modelling with rational numbers, percentages and financial contexts.
• Measurement — mathematical modelling with ratios and rates in practical and financial contexts.
• Algebra — creating and using linear relationships to model applied financial problems.
• Algebra — selecting representations, interpreting solutions and reviewing the appropriateness of a model.

Lesson structure (90 minutes)

1. 0–8 min · Hook and prior knowledge. Open with the budget challenge hook and display the question: “Is the cheapest item always the best value for a sustainable community?” Present a community garden budget of $2,000 and a supplier item with a discount, GST and delivery fee. Students estimate the final cost, discuss what information is needed, and share one percentage strategy they remember.

2. 8–23 min · Explicit teaching. Use the percentage calculation examples to model percentage change with a clear sequence: find the percentage amount, decide whether to add or subtract, and check that the answer is sensible. Work through $240 with a 15% discount, $180 with a 20% mark-up, and $500 plus 10% GST. Then model a 5% contingency allowance on a subtotal, emphasising that it is added to the cost and represents possible unexpected expenses. Students calculate each example on mini whiteboards and explain their operation to a partner.

3. 23–32 min · Guided comparison. Distribute the sustainable budget planning worksheet. With the six students working in three pairs, complete the first supplier comparison together. Students identify the unit cost, quantity, percentage change, delivery charge and final cost before deciding which supplier offers better value. Pause for a peer check: one student calculates while the other explains and verifies each step, then they swap roles.

4. 32–62 min · Budget design investigation. Provide supplier catalogues and the community brief: design a small sustainable space with seating, native plants, water-saving equipment, lighting and recycling facilities, staying within a $2,000 budget. Students select quantities and suppliers, record assumptions in the worksheet, and calculate discounts, GST or a contingency allowance where shown. They may use a spreadsheet template to enter formulas and totals, or complete the table by hand. Confer with each pair, asking: “What does this percentage represent?”, “How do you know your total is reasonable?” and “What have you assumed about delivery or GST?” Students must retain enough evidence to explain their final choices.

5. 62–75 min · Scenario testing and review. Return to the scenario-testing prompts. Students test one change, such as a 10% rise in timber prices, a different supplier’s delivery fee, or removing a discount. They update their table or spreadsheet and determine whether the design remains within budget. Each pair writes a brief review identifying one strength, one limitation and one change they would make to improve value or sustainability.

6. 75–85 min · Present and critique. Each pair gives a two-minute budget briefing: total cost, key percentage calculations, chosen supplier options and recommendation. The other students use the worksheet’s peer-check prompts to identify one calculation they agree with and ask one question about an assumption. Students revise one figure or statement if peer feedback reveals an error.

7. 85–90 min · Exit reflection. Show the plenary and exit questions. Students independently answer: “An item costs $360 and is discounted by 15%. What is the sale price?” and “Why might a 5% contingency be appropriate in a community budget?” Collect responses and ask students to rate their confidence from 1 to 5.

Resources

• the sustainable budgeting slide deck
• the sustainable budget planning worksheet
• Supplier catalogues with prices, quantities, discounts, GST and delivery information
• Spreadsheet template and devices, where available
• Mini whiteboards, markers and calculators
• Budget brief showing the $2,000 limit
• Money manipulatives and percentage grids
• Coloured pens for peer checking

Assessment

• Listen to mini-whiteboard explanations and check whether students choose the correct operation for a discount, mark-up, GST or contingency.
• During the investigation, check tables or formulas for accurate totals, units, assumptions and sensible interpretation of results.
• Use the presentation, peer feedback and exit questions to assess calculation accuracy, comparison of options and communication of a justified recommendation.

Differentiation

• Support students with percentage grids, money manipulatives, a reduced-cost dataset, calculators and the worked examples from the slides. Break each calculation into “percentage amount” and “new total” steps.
• Provide a partially completed worksheet, highlighted key information and sentence starters such as “The cost increases because…” and “Supplier A is better value because…”.
• Pair students strategically and assign rotating roles: calculator, recorder, checker and spokesperson. Read catalogue information aloud and clarify financial vocabulary for EAL learners.
• For advanced learners, model multiple scenarios, include two percentage changes in sequence, compare total and per-unit costs, and recommend the best value option with evidence while discussing limitations of the model.