Math Detectives: Solving Addition Stories Within 20

Materials Needed

• 20 small objects for counting, such as buttons, blocks, coins, or cereal pieces
• Paper and pencil or crayons
• Number cards from 0 to 20, homemade or printed
• Two small bowls, cups, or paper plates
• Optional: toy animals, stuffed animals, or a drawing of a “detective” character

Learning Objectives

By the end of the lesson, the learner will be able to:

• Explain that addition means putting groups together.
• Use objects, drawings, or a number line to solve addition problems within 20.
• Write an addition equation to match a simple story problem.
• Explain how they know an answer is correct.

Success Criteria

I can:

• Make two groups of objects.
• Put the groups together and count the total.
• Write a number sentence such as 6 + 4 = 10.
• Tell or show how I solved the problem.

Introduction: The Mystery of the Missing Snacks

Hook: Tell the learner:

“A detective is preparing snacks for a group of toy animals. There are 5 crackers in one bowl and 3 crackers in another bowl. How many crackers are there altogether?”

Invite the learner to predict the answer before solving. Ask:

• What does “altogether” mean?
• What could we do to find the answer?
• How could we prove our answer?

Explain:

“Today we are going to be math detectives. We will put groups together, count carefully, and write addition equations to solve number mysteries.”

Body: Learn and Practice

Part 1: I Do — Teacher or Adult Models Addition

Place 5 objects in one bowl and 3 objects in the other bowl.

1. Point to the first group and say, “This group has 5.”
2. Point to the second group and say, “This group has 3.”
3. Push both groups together.
4. Count all the objects aloud: “1, 2, 3, 4, 5, 6, 7, 8.”
5. Write the equation: 5 + 3 = 8.

Explain the parts of the equation:

• 5 is the first group.
• + means “put together” or “add.”
• 3 is the second group.
• = means “is the same as.”
• 8 is the total.

Quick Check: Hide the objects and ask, “What did we add? What was the total? What equation did we write?”

Part 2: We Do — Solve Together

Use the two bowls and objects to solve each problem together. Encourage the learner to build, count, and explain each answer.

1. Put 2 objects in one bowl and 4 in the other. Write 2 + 4 = 6.
2. Put 7 objects in one bowl and 1 in the other. Write 7 + 1 = 8.
3. Put 6 objects in one bowl and 5 in the other. Write 6 + 5 = 11.

After each problem, ask:

• How many were in the first group?
• How many were in the second group?
• What is the total?
• How can we check the answer?

Think-Pair-Share or Talk-and-Draw

Ask the learner to think quietly about this problem: 4 birds are sitting in a tree. 3 more birds fly in. How many birds are there now?

The learner may choose one way to show the answer:

• Build it with objects.
• Draw the birds.
• Use a number line.
• Explain the answer aloud.

Have the learner share the strategy and write the matching equation: 4 + 3 = 7.

Part 3: You Do — Math Detective Missions

Explain:

“Now you are the detective. Solve each mystery. You may use objects, drawings, fingers, a number line, or mental math.”

1. Animal Mission: There are 3 toy dogs and 6 toy cats. How many animals are there altogether?
2. Snack Mission: You have 8 crackers and receive 4 more. How many crackers do you have now?
3. Nature Mission: You find 5 red leaves and 7 yellow leaves. How many leaves did you find?

For each mission, the learner should:

1. Underline or identify the two groups.
2. Choose a strategy.
3. Solve the problem.
4. Write an equation.
5. Explain how the answer was found.

Creative Choice Activity

Let the learner choose one project:

• Make a comic: Draw a short story with two groups joining together. Add an equation beneath the picture.
• Create a pretend shop: Draw or gather items for sale. Make two groups of items and find the total number of items.
• Invent a mystery: Write or dictate an addition story for someone else to solve.

Formative Assessment

Check understanding throughout the lesson by asking the learner to:

• Build an addition problem with objects.
• Point to the two groups and the total.
• Explain what the plus sign means.
• Match a drawing to an equation.
• Find and correct an error, such as 3 + 2 = 4.

Summative Assessment: Final Detective Challenge

Give the learner this problem:

“A park has 9 children playing on the playground. Then 6 more children arrive. How many children are playing at the park now?”

Ask the learner to complete all four steps:

1. Show the two groups using objects, a drawing, or a number line.
2. Find the total.
3. Write the equation: 9 + 6 = 15.
4. Explain the strategy used.

Assessment Guide

• Developing: Needs help making the groups or counting the total.
• Progressing: Solves the problem with objects or drawings and writes part of the equation.
• Meeting the Objective: Correctly solves the problem, writes the equation, and explains the strategy.
• Extending: Creates a second story with the same equation or solves the problem in two different ways.

Differentiation and Support

For Learners Who Need More Support

• Begin with totals up to 10.
• Use larger objects that are easy to move and count.
• Provide a ten-frame or number line.
• Have the learner touch each object while counting.
• Use sentence frames: “I had ___ and added ___. Now I have ___.”

For Learners Ready for a Challenge

• Solve addition problems within 30.
• Find two different equations with the same total, such as 5 + 5 = 10 and 7 + 3 = 10.
• Write a problem with a missing number, such as 8 + ___ = 13.
• Explain how counting on can be faster than counting every object.

Conclusion: Close the Case

Invite the learner to complete these statements:

• “Addition means…”
• “The plus sign tells me to…”
• “One strategy I used was…”
• “I know my answer is correct because…”

Review the key idea:

“Addition helps us find how many there are altogether. We can put groups together, count the total, and write an equation to show our thinking.”

Exit Ticket: Ask the learner to solve 7 + 8 and show the answer in one way besides writing the equation.

Optional Real-World Connection

During the day, look for two groups that can be combined, such as socks in two piles, fruits in two bowls, or books on two shelves. Ask the learner to count both groups, find the total, and describe the matching addition equation.