Decimal Detectives: Add It Up!
Students work in small teams to solve a snack or supply budget challenge with money amounts written as decimals. They add and subtract to find the best purchase plan, show clear math work, and explain why their answer makes sense. Teams create a final budget plan with labels, equations, and a short explanation of their strategy. Then they take part in a Decimal Detectives Gallery Walk, where classmates rotate to view each plan and leave one compliment and one improvement note. Before the exit ticket, teams revise their explanation using the feedback they received. End with a whole-class circle-up reflection where students share one way they checked their answers and one way they supported a classmate.
Steps
| # | What teachers do | Time |
|---|---|---|
| 1 | Launch the Decimal Detectives Budget Blitz by presenting the snack or supply budget challenge and asking, “How can we work together to use decimals to solve a challenge and explain our thinking?” | 5 min |
| 2 | Review how to align decimal points when adding and subtracting money amounts, and model checking an answer with estimation or the inverse operation. | 6 min |
| 3 | Assign team roles and guide teams to compare possible purchases, add item costs, subtract totals from the budget, and select the best plan without exceeding the spending limit. | 13 min |
| 4 | Direct each team to create a final budget plan that includes labeled items, decimal prices, equations, the total cost, money remaining, and a short explanation of why the plan makes sense. | 6 min |
| 5 | Facilitate a Decimal Detectives Gallery Walk in which students examine each displayed plan and leave one specific compliment and one helpful improvement note. | 6 min |
| 6 | Have teams revise their explanation using peer feedback, and then give an individual exit ticket asking students to solve one decimal problem and describe how they know their answer makes sense. | 6 min |
| 7 | Gather students in a circle and invite each student to share one way they checked an answer and one way they supported a classmate. | 3 min |
| Total | 45 mins |
Preparation
- Prepare one realistic snack or classroom-supply price list for each team, with 8–10 items priced to the hundredths place, plus a fixed budget and a clear purchasing goal. Check in advance that students can create several possible plans and determine a best choice by adding and subtracting decimals.
- Gather chart paper or large paper, pencils, markers, and play money if available. Each team’s final plan should include selected items, labeled prices, decimal equations, total cost, money remaining, and a brief explanation of why the plan meets the goal.
- Arrange students in teams of three or four and prepare participation roles such as price reader, calculator, accuracy checker, and explanation writer; roles may rotate so every student contributes. Provide decimal place-value charts or lined-up decimal examples for students who need additional support.
- Set up a gallery-walk path and place two small notes beside each display for every visiting team. Explain that each group will leave one specific compliment about the math or explanation and one respectful suggestion for improvement.
- Prepare an individual exit ticket with one real-world decimal addition or subtraction problem and space for students to explain how they know the answer is reasonable. Reserve the final minutes for revision, the exit ticket, and a class circle in which each student can share how they checked an answer and supported a classmate.
Standards
- [Pennsylvania] CC.2.1.5.B.2 - Extend an understanding of operations with whole numbers to perform operations including decimals.
Competencies
- Critical Thinking & Problem Solving - Students consider a variety of innovative approaches to address and understand complex questions that are authentic and important to their communities.
Teacher Notes
Prepare realistic snack or classroom-supply price lists, a budget limit, chart paper, markers, and sticky notes; ensure each challenge requires both decimal addition and subtraction and allows more than one reasonable plan. Use the essential question to launch the 45-minute lesson, then allot about 20 minutes for team planning, 8 minutes for the gallery walk—a rotation in which students view displayed plans and leave one compliment and one improvement note—5 minutes for revision, and 5 minutes each for the exit ticket and reflection circle. Prompt students to align decimal points, estimate totals, compare possible plans, label equations, and explain why their choice fits the budget; provide play money, a place-value chart, or fewer price options as needed. Assess the team budget plan for accurate operations and clear reasoning, then collect an individual exit ticket with one decimal problem and an explanation of why the answer makes sense before students share how they checked their work and supported a classmate.
Student Instructions
Work with your team to answer: “How can we work together to use decimals to solve a challenge and explain our thinking?” Use the given budget and price list to compare snack or supply combinations, add and subtract decimal amounts, and check that your total stays within budget. Create a team budget plan that labels each item and price, shows equations and calculations, identifies your best purchase plan, and explains why your answer makes sense. Display your plan for the Decimal Detectives Gallery Walk, review classmates’ work, and leave each team one specific compliment and one helpful improvement note; then use the feedback to revise your explanation. Complete an exit ticket by solving one decimal problem and explaining how you know your answer is reasonable, then join the class circle to share one way you checked your work and one way you supported a teammate.
Success Criteria
- I can accurately add and subtract decimal money amounts in a real-world budget problem.
- I can create a budget plan that stays within the spending limit and clearly labels each purchase and cost.
- I can show equations and use estimation or an inverse operation to check that my answer makes sense.
- I can explain my team’s strategy using clear mathematical reasoning.
- I can give a classmate one specific compliment and one helpful improvement note, then use feedback to revise my team’s explanation.
- I can describe one way I supported a classmate during the challenge.
Differentiation
- Provide a place-value mat labeled ones, tenths, and hundredths so students can align decimal points and model each money amount before calculating.
- Assign rotating team roles—price reader, calculator, checker, and explanation writer—so every student contributes to the budget plan.
- Offer a three-step checking guide: estimate the total, solve using a second method, and compare the total with the budget.
- Preteach and display budget vocabulary with examples and visuals: budget, cost, total, difference, estimate, and change.
- Provide sentence frames such as, “We chose ___ because ___,” “Our total is ___,” and “Our answer makes sense because ___.”
- Allow students to discuss strategies in their home language before sharing in English and to use labeled equations or drawings in their explanation.
- Ask students to create a second valid purchase plan and compare which plan uses more of the budget or provides more items.
- Add a constraint, such as including at least three different items or staying within $1.00 of the budget without exceeding it.
- Have students analyze a sample calculation containing a decimal-alignment error, correct it, and explain how the error affected the total.
- Provide enlarged, uncluttered price lists with one item and price per line, highlighted decimal points, and reduced visual distractions.
- Allow extra processing time, brief movement breaks, and a quieter workspace during calculation, revision, or the exit ticket as specified in a student’s IEP or 504 plan.
- Allow students to dictate their explanation to a peer or adult, use speech-to-text, or give an oral explanation while still showing the required decimal equations.