Decimal Detectives

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Overview

Duration: 60 minutes
Class size: 30 pupils
Subject: Mathematics
Year group: Year 5
UK National Curriculum Objective:
Number - number and place value; Number - addition and subtraction

Curriculum Links:

• Year 5 National Curriculum: Pupils should be taught to:
• Solve problems involving number up to three decimal places.
• Add and subtract decimals, including a mix of whole numbers and decimals, decimals with the same number of decimal places and with different numbers of decimal places.
• Use rounding to check answers to calculations and determine, in the context of a problem, levels of accuracy.

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Learning Objectives

By the end of the session, pupils will be able to:

• Fluently add and subtract decimals with the same and differing numbers of decimal places
• Apply these skills to real-world word problems
• Reason about their methods and explain their strategies using correct mathematical vocabulary

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Success Criteria

✔ I can align decimal points correctly when calculating
✔ I can use inverse operations to check my answers
✔ I can explain my method to a partner or the class
✔ I can solve real-life problems involving money or measurement using decimals

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Vocabulary

• Decimal point
• Tenths, Hundredths
• Whole number
• Estimate
• Difference
• Total
• Inverse operation
• Calculate

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Resources

• Smartboard or whiteboard
• “Decimal Detectives” themed worksheet (provided below)
• Place value grid boards and counters (enough for pairs)
• Whiteboards and pens for each student
• Mini plastic coins or cut-outs for role play (optional extension)

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Prior Learning

Children should already understand:

• The place value of digits in decimal numbers up to two decimal places
• How to add and subtract whole numbers using formal written methods
• Strategies for mental addition/subtraction

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Starter (10 minutes) – Decimal Riddle Warm-up

Activity Name: "Who Am I?" Decimal Edition

• Project or write this riddle on the board:

  "I am a number with a 7 in the tenths place and a 3 in the hundredths. My whole number is 5. Who am I?"

• Ask the class to solve individually on mini whiteboards. Take several answers and discuss reasoning.

• Repeat the riddle format with a couple more examples, asking pupils to create one with their talk partner.

Purpose: Reinforce decimal place value and use of correct vocabulary from the start.

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Main Input (15 minutes) – The Power of Decimal Alignment

Teacher-led demonstration:

1. Explain that today we are becoming "Decimal Detectives" and our mission is to solve mysteries using decimal knowledge.
2. Display the following example on the board:

   3.45
   

• 2.67

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Ask pupils to identify the importance of the decimal points lining up.
3. Move to a contrasting example:

4.2

• 3.56

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Ask pupils: "What’s different here? Why is lining up important?"

**Model the working out of both examples using both column method and place value grids. Show regrouping if necessary.**

**Key Questions:**

- How did you know where to put each number?  
- Why is it important to align decimal points?  
- Can we estimate to check the answer makes sense?

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## Independent Activity (20 minutes) – **"Decimal Detectives" Mystery Sheet**

Distribute the custom worksheet. Pupils complete independently or in guided pairs, depending on need.

### 🔍 Decimal Detectives Worksheet:

**Section A – Column Calculations**
*(Complete using the column method)*

1. 5.3 + 2.7 =  
2. 6.45 − 3.2 =  
3. 2.76 + 4.123 =  
4. 9.1 − 2.07 =  

**Section B – Estimate and Calculate**

Estimate first, then solve:

5. 7.8 + 2.14 ≈ _______ → Exact answer: ________  
6. 12.34 − 6.9 ≈ _______ → Exact answer: ________

**Section C – Real-life Word Problems**

7. **Ella bought an ice cream for £2.35 and a drink for £1.8. How much did she spend altogether?**

8. **A bag of potatoes weighs 3.75 kg. Joseph uses 1.8 kg for dinner. How much is left?**

9. **A movie starts at 14:00 and lasts for 1.45 hours. What time does it finish?**

10. **Ravi wants to buy a toy costing £9.99. He has £6.75. How much more does he need?**

🎯 **Challenge Extension** (Optional):

> The total cost of three items is £12.87. Two of the items cost £4.50 and £3.69. What is the missing price?

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## Plenary (10 minutes) – **Decimal Detective’s Report**

Group "Detective Debrief":

- Ask different pupils: “Tell us one strategy you used today to solve a decimal calculation.”  
- “How did you check your answer?”  
- “Which problem did you find tricky, and how did you solve it?”  
- Choose a pupil to be the “Chief Detective” and summarise what they learned today.

Finish with one final riddle:
> I am a number between 5 and 6. My hundredths digit is 9, my tenths digit is 8. Who am I?

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## Assessment Opportunities

- Observation of pupil methods during the main activity  
- Use of mathematical reasoning in estimations  
- Accurate use of written methods in worksheet answers  
- Quality of vocabulary used during plenary  

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## Differentiation

**Support**:
- Use place value grid mats and counters to visualise  
- Allow use of calculators to check answers post-calculation  
- Remove hundredths from some calculations where needed

**Challenge**:
- Provide word problems involving mixed operations  
- Introduce numbers with three decimal places (e.g. 4.527 – extension only)

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## Reflection & Next Steps

**Teacher notes post-lesson** (space for reflection):

- What strategies worked best for pupil understanding?
- Which problems promoted the deepest thinking?
- Plan to extend to multiplication/division of decimals next week?

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## Homework (Optional Extension)

Create your own “Decimal Detective” problem involving adding or subtracting decimals. Include an answer and a method explanation. Best ones get added to the classroom display!

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## ‘Wow’ Factor Twist 🎥

Set up a small 'Decimal Detective Agency' corner in the classroom. Pupils visit for clues (word problems glued into envelopes or folders) and they 'solve' them to earn star badges or certificates. Consider "interviewing" your top detective at the end of the week in a video format or with a display write-up.

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Let the decimal sleuthing begin! 🕵️‍♂️