# Lesson 8

Add and Subtract Within 100

## Warm-up: Number Talk: Add by Place (10 minutes)

### Narrative

The purpose of this Number Talk is to elicit strategies and understandings students have for adding by place. These understandings help students develop fluency and will be helpful later in this lesson when students will need to be able to consider place value when they add and subtract within 100.

### Launch

- Display one expression.
- “Give me a signal when you have an answer and can explain how you got it.”
- 1 minute: quiet think time

### Activity

- Record answers and strategies.
- Keep expressions and work displayed.
- Repeat with each expression.

### Student Facing

Find the value of each expression mentally.

- \(9 + 5\)
- \(20 + 30\)
- \(29 + 35\)
- \(229 + 435\)

### Student Response

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### Activity Synthesis

- “How is each sum related to the others?” (Sample response: The third expression is like the first and the second because you have to add \(9+5\) and \(20+30\) if you add by place.)

## Activity 1: Heads Up, Add and Subtract within 100 (15 minutes)

### Narrative

The purpose of this activity is to give students practice adding and subtracting within 100. Students work in groups of 3. One student adds to find the sum of 2 numbers. The other students add or subtract to find the unknown addend when one addend and the value of the sum are known. Although the majority of sums will fall within 100, some students may create sums greater than 100.

*MLR8 Discussion Supports.*Synthesis: Display question starters to support small group discussion about favorite role in the game: “Why did you . . . ?”, “Can you say more about . . . ?”

Advances: Speaking, Conversing

*Representation: Develop Language and Symbols.*Support understanding of the problem by inviting students to act it out using manipulatives. For example, one student can build the numbers with base-ten blocks and the other students can complete the action of composing or decomposing to find and prove the answer.

*Supports accessibility for: Conceptual Processing, Attention*

### Required Materials

Materials to Copy

- Heads Up - Add and Subtract within 100 Number Cards

### Required Preparation

- Create a set of cards from the blackline master for each group of 3.

### Launch

- Groups of 3
- Give students number cards.

### Activity

- “We are going to play a game called Heads Up.”
- Demonstrate with 2 students.
- “Players A and B pick a card and put it on their foreheads without looking at it.”
- “I am Player C. My job is to find the value of the sum and tell my group.”
- “Players A and B use the other player’s number and the value of the sum to determine what number is on their head.”
- “Finally, each player writes the equation that represents what they did.”
- Demonstrate writing an equation for each of the players.
- “After each round switch roles and play again.”
- 10 minutes: small-group work time

### Student Facing

Write an equation that shows the value you found for each round you play.

Round 1:

Round 2:

Round 3:

### Student Response

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### Activity Synthesis

- “Which role did you like best—Player C or Player A and B? Explain.” (I liked to be Player C because I like to add. I liked being Player A or B because I could add or subtract. I liked being Player A or B because I could practice subtracting.)

## Activity 2: Centers: Choice Time (20 minutes)

### Narrative

The purpose of this activity is for students to choose the center activity that will most help them build their fluency for addition and subtraction within 100. These two centers were introduced in previous units and focus on adding and subtracting within 100:

- Five in a Row
- Target Numbers

### Required Materials

Materials to Gather

### Required Preparation

- Each group of 2 needs access to the cards used in the previous activity.
- Gather materials from previous centers:
- Five in a Row, Stage 6
- Target Numbers, Stages 3–5

### Launch

- Groups of 2
- “Now you will choose from different center activities we learned. You may also choose to continue with Heads Up.”
- Display the student page.
- “Think about which activity you would like to do first.”
- 30 seconds: quiet think time

### Activity

- Invite students to work at the center of their choice.
- 10 minutes: center work time
- “Think about what center you would like to do now.”
- 30 seconds: quiet think time
- Invite students to work at the center of their choice.
- 10 minutes: center work time

### Student Facing

Choose a center.

Five in a Row

Target Numbers

### Activity Synthesis

- “What did you like about the activities you worked on today?”

## Lesson Synthesis

### Lesson Synthesis

“Share your work from the cool-down with your partner. Then we will share with the whole class.”

## Cool-down: Practice for Fluency (5 minutes)

### Cool-Down

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