Add and Subtract Context

Overview

In this seventh lesson of the 12-lesson unit Decimal Fractions and Operations, students apply decimal addition and subtraction to familiar financial and measurement situations. They build on place-value understanding and written calculation strategies, then communicate a complete solution using an operation, accurate calculation, units and an explanation.

Learning intentions

• WALT solve decimal addition and subtraction problems in context.
• WALT identify the operation needed and represent the problem mathematically.
• WALT calculate accurately using an efficient strategy.
• WALT communicate and justify a solution clearly.

Success criteria

• I can identify whether a problem requires addition or subtraction.
• I can represent the problem with an equation, diagram or labelled working.
• I can calculate accurately and check whether my answer is reasonable.
• I can include appropriate units and explain my strategy.

Curriculum links

• Mathematics and Statistics — Decimal Fractions and Operations: applying decimal knowledge to meaningful problems.
• Mathematics and Statistics — Mathsteasers: using higher-order thinking questions to deepen mathematical understanding.
• Mathsteasers / Alignment: connecting challenge problems with classroom mathematics content.
• Mathematical practice: reasoning, communicating mathematical thinking and selecting efficient strategies.

Lesson structure (60 minutes)

1. 0–5 min · Hook and notice. Open with the hook and learning intention slides showing a supermarket receipt with prices such as $3.75, $2.40 and $5.65, and ask: “Can you find the total and the change from $20 without using a formal algorithm?” Students estimate, discuss whether each calculation requires addition or subtraction, and share an initial strategy.

2. 5–15 min · Model the process. Use the worked-example slides to model: “A family buys fruit for $4.85 and bread for $3.70. How much do they spend altogether?” Think aloud while identifying the operation, writing an equation, aligning decimal points, calculating, adding the dollar sign and checking the estimate. Briefly model a subtraction example involving change, highlighting the meaning of the answer and the unit.

3. 15–22 min · Guided reasoning. Display the two guided problems in the guided-practice slides. Students solve on mini-whiteboards, first estimating and then calculating. Pause after each problem for students to explain why they chose addition or subtraction; check alignment of decimal points, regrouping and units before revealing the answers.

4. 22–42 min · Collaborative problem solving. Distribute the decimal problems worksheet to pairs. Students complete the one-step problems first, then select and solve multi-step budgeting and measurement problems. Require each pair to underline key information, circle the question, write an equation, show working and compose a written answer. Circulate using prompts: “What does the question ask?”, “Which operation matches the context?”, “What unit belongs in your answer?” and “How could you check it?” Students compare strategies with another pair and revise unclear explanations.

5. 42–53 min · Open-ended challenge and sharing. Show the challenge and discussion slides. Students choose one challenge: create a shopping budget using a fixed amount and at least three items, or design a measurement problem with a total and a difference. More than one answer must be possible. Students solve their own problem, swap with a partner, and explain how they know the partner’s solution is valid. Invite two pairs to present different strategies, including one using compensation or mental calculation.

6. 53–60 min · Plenary and exit check. Return to the reflection and exit-question slide and ask students to complete this individual response on the back of their worksheet: “A tank contains 12.5 L. It uses 3.75 L in the morning and 2.4 L later. How much remains? Show an equation, calculation, units and one checking method.” Students finish with one sentence: “The strategy I will use next time is …”

Resources

• the complete decimal context slide deck
• the decimal problems worksheet
• Mini-whiteboards, pens and erasers
• Calculators for checking only, if appropriate
• Projector or interactive display
• Pencils, rulers and highlighters
• Pairing cards or a planned mixed-ability seating arrangement

Assessment

• Listen during the hook and guided practice for accurate operation choice, estimation and mathematical vocabulary.
• Confer with pairs during the worksheet task, checking representation, decimal alignment, calculation, units and explanation.
• Use the individual exit response to identify students needing further support with operation selection, regrouping, multi-step reasoning or communicating solutions.

Differentiation

• Provide one-step problems before multi-step tasks; allow students to progress when they can identify the operation and explain the context.
• Support readers with the worksheet’s highlighted key information, vocabulary read aloud, partner reading and sentence starters such as “I chose ___ because …”.
• Offer an operation prompt card or a two-column organiser labelled “What do I know?” and “What do I need to find?” Avoid giving the operation immediately; use prompts that preserve reasoning.
• Encourage collaborative problem solving with defined roles: reader, calculator, checker and explainer. Provide place-value grids or lined decimal templates for students who need help aligning numbers.
• Extend advanced learners through open-ended budgeting or measurement challenges with more than one possible answer. Require them to justify constraints, compare two methods and explain why their answer is reasonable.