# Lesson 9

Add and Subtract Within 100

## Warm-up: Which One Doesn’t Belong: Tens and Ones (10 minutes)

### Narrative

### Launch

- Groups of 2
- Display the image.
- “Pick one that doesn’t belong. Be ready to share why it doesn’t belong.”
- 1 minute: quiet think time

### Activity

- “Discuss your thinking with your partner.”
- 2–3 minutes: partner discussion
- Share and record responses.

### Student Facing

Which one doesn’t belong?

### Student Response

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### Activity Synthesis

- “Let’s find at least one reason why each one doesn’t belong.”

## Activity 1: Sort and Find the Value (15 minutes)

### Narrative

The purpose of this activity is for students to match expressions to base-ten diagrams. Students then choose 2 of the expressions to find the value of, using any method that makes sense to them. Some of the expressions do not require composing or decomposing a ten. When students match expressions with diagrams they are making use of base ten structure and the meaning of operations (MP7).

*MLR8 Discussion Supports.*Students should take turns finding a match and explaining their reasoning to their partner. Display the following sentence frames for all to see: “I noticed _____, so I matched . . . .” Encourage students to challenge each other when they disagree.

*Advances: Conversing, Representing*

### Required Materials

### Required Preparation

- Create a set of cards from the blackline master for each group of 2–3.

### Launch

- Groups of 2–3
- Give each group a set of cards and access to base-ten blocks.

### Activity

- “Each group will get a set of cards. Match each expression to a diagram. After you have found a match, explain to your partner why you believe they go together.”
- “After you have found all of the matches, choose 1 addition and 1 subtraction expression. Find the value of each expression in a way that makes sense to you.”
- 15 minutes: partner work time
- Monitor for students who choose expressions that do not involve composing or decomposing a ten.

### Student Facing

- Match each expression to a base-ten diagram.
- Choose 1 addition expression and find the value of the sum.
- Choose 1 subtraction expression and find value of the difference.

### Student Response

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### Activity Synthesis

- “Which expressions did you choose to solve? Why?” (I chose \(35 + 42\) because it was easy for me. I knew that I could just add the ones and then add the tens.)
- “How could you tell if you would need to compose or decompose a ten?” (I could see that \(5 + 2 = 7\), so I knew I wouldn’t need to compose a ten.)

## Activity 2: Add or Subtract (20 minutes)

### Narrative

The purpose of this activity is for students to add and subtract within 100 using the methods that make sense to them. Throughout the activity students share their methods for adding and subtracting and compare their method with others (MP3).

This activity uses *MLR7 Compare and Connect*. Advances: representing, conversing

*Representation: Internalize Comprehension.*Synthesis: Invite students to identify which details were most important to solve the problem. Display the sentence frame, “The next time I need to find the value of an expression, I will pay attention to . . . .“

*Supports accessibility for: Conceptual Processing, Memory, Language*

### Required Materials

Materials to Gather

### Launch

- Groups of 2–3
- Give each group access to base-ten blocks.

### Activity

- “Find the value of each expression. Show your thinking using drawings, numbers, or words.”
- “You can use the base-ten blocks if they help. Make sure you show your thinking on paper.”
- 5 minutes: independent work time

**MLR7 Compare and Connect**

- “Now, talk with your group about how you found the value of the expressions. What is the same? What is different?”
- “Create a visual display that shows your thinking about 1 of the expressions. Show the work of all of the group members for the same expression so others can look for things that are the same or different. You may want to include details such as notes, diagrams, drawings, etc. to help others understand your thinking.”
- 5 minutes: partner discussion

### Student Facing

Find the value of each expression. Show your thinking. Use blocks if it helps.

- \(27 + 47\)
- \(55 - 27\)
- \(36 + 38\)
- \(82 - 39\)

### Student Response

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### Advancing Student Thinking

Students may not see the connections between their methods and other students’ methods. Consider asking:

- “How are the methods represented differently in each display?”
- “How did each group find the same value when they used such different methods?”

### Activity Synthesis

- 5–7 minutes: gallery walk
- “What was the same about how _____ found the value and _____ found the value?” (In the first problem, _____ and _____ both added the ones and then added the tens and combined the two sums. \(7 + 7 = 14\), \(20 + 40 = 60\), \(14 + 60 = 74\))
- “What is different about how _____ represented their thinking and ______ represented theirs?” ( _____ used a diagram and crossed out the ones and then decomposed a ten. Then _____ crossed out the rest of the ones and the tens. _____ wrote equations to show each step.)

## Lesson Synthesis

### Lesson Synthesis

“In this unit, you added and subtracted within 100 using different methods, tools, and representations.”

“What is something new you've learned about addition or subtraction?”

“What is something new you've learned about ways to add or subtract from another classmate?”

## Cool-down: Find the Value Your Way (5 minutes)

### Cool-Down

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## Student Section Summary

### Student Facing

In this section, we practiced subtracting two-digit numbers. We learned that when there are not enough ones to subtract by place, you can decompose 1 ten for 10 ones. We used base-ten blocks and base-ten diagrams to show our thinking.

\(63 - 18\)