Mystery Number Detectives

Amber Keith

Tier 2

Lesson Plan

Mystery Math Investigation

Students will be able to solve for unknown numbers in addition and subtraction equations using inverse operations.

Understanding inverse operations helps students grasp the fundamental relationship between addition and subtraction, building a strong foundation for more complex math problems.

Audience

1st Grade Students

Time

40 minutes

Approach

Engaging small group activity and independent practice.

Materials

Who's the Missing Number?, Clue Card Hunt, Detective's Logbook, Pencils, and Whiteboards and markers (optional)

Prep

Prepare Materials

15 minutes

Review the Mystery Math Investigation Lesson Plan and all supporting materials.
Print and cut out the 'clue cards' for the Clue Card Hunt.
Make copies of the Detective's Logbook for each student.
Ensure the Who's the Missing Number? Slide Deck is ready for projection.

Step 1

Warm-Up: Brain Teaser

5 minutes

Begin with a quick warm-up on the board: 'I'm thinking of a number. If you add 3 to it, you get 7. What's my number?' Discuss how they solved it. Introduce the idea of being 'mystery number detectives'.

Step 2

Introducing Inverse Operations

10 minutes

Use the Who's the Missing Number? Slide Deck to explain inverse operations. Show examples of how addition can undo subtraction, and subtraction can undo addition, to find a missing number. Work through 2-3 examples together as a class.

Step 3

Clue Card Hunt Activity

15 minutes

Divide students into small groups (3-4 students per group). Distribute the Clue Card Hunt Activity cards around the classroom or at group stations. Each card will have a 'mystery number' problem. Students work together to solve the problems, using inverse operations. They record their answers in their Detective's Logbook. Circulate to provide support and clarification.

Step 4

Share and Discuss

7 minutes

Bring the class back together. Have groups share some of the mystery numbers they discovered and explain their strategies. Emphasize how they used the opposite operation to 'unlock' the secret number.

Step 5

Cool-Down: Quick Check

3 minutes

Distribute the Detective's Logbook and have students complete the last problem independently as an exit ticket. Collect to assess understanding.

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Slide Deck

Who's the Missing Number?

🕵️‍♂️ Be a Math Detective! 🕵️‍♀️

Sometimes numbers like to play hide-and-seek!

Today, we'll learn how to find them!

Welcome students and get them excited about being 'Mystery Number Detectives'. Explain that today they'll learn a special trick to find numbers that are hiding.

Opposites Attract (and Help!)

In real life, we have opposites:

Open ↔️ Close
On ↔️ Off

In math, we have opposites too!

Addition is the opposite of Subtraction

These opposites are called inverse operations!

Introduce the idea of opposite actions. Give examples like 'open' and 'close', 'on' and 'off'. Then transition to math opposites: addition and subtraction.

Catching a Missing Number: Addition

Let's say we have:

3 + ? = 7

The '?' is our mystery number!

To find it, we can use the opposite of addition...

Subtraction!

7 - 3 = 4

So, 3 + 4 = 7! Our mystery number is 4!

Show a simple addition problem with a missing addend. Explain how subtracting helps find the missing number. Work through an example together.

Catching a Missing Number: Subtraction

What if we have:

9 - ? = 5

That '?' is playing tricky!

To find it, we can use the opposite of subtraction...

Addition!

5 + ? = 9 (Think: what do I add to 5 to get 9?)

Alternatively, 9 - 5 = 4

So, 9 - 4 = 5! Our mystery number is 4!

Show a simple subtraction problem with a missing number. Explain how adding helps find the missing number. Work through an example together.

Your Turn, Detectives!

Solve this mystery:

? + 6 = 10

What's the missing number?

Hint: Use the opposite operation!

Provide a practice problem for students to try on their whiteboards or with a partner. Give them a minute, then reveal the answer and explain the steps.

Case Solved!

If ? + 6 = 10

We use subtraction!

10 - 6 = 4

So, 4 + 6 = 10! The missing number is 4! Good job, detectives!

Reveal the solution to the previous slide's problem. Reinforce the concept of using the inverse operation.

Time for a Clue Card Hunt!

Now that you're expert inverse operation detectives, it's time to put your skills to the test!

Your mission: Find the Clue Card Hunt cards hidden around the room and solve the mystery numbers using your Detective's Logbook.

Transition to the next activity. Explain that they will now go on a 'Clue Card Hunt' to find more mystery numbers.

Activity

Clue Card Hunt: Find the Missing Number!

Instructions for Detectives:

Work with your group to find the Clue Cards hidden around the classroom.
Each card has a mystery number problem.
Use your detective skills (and inverse operations!) to find the missing number.
Record the problem and your answer in your Detective's Logbook.

What's the mystery number?

Worksheet

Detective's Logbook

Name: ________________________

Date: ________________________

Your Mission: Find the Mystery Numbers!