# Lesson 7

Add and Subtract Within 1,000

## Warm-up: Which One Doesn’t Belong: Add and Subtract Within 100 (10 minutes)

### Narrative

This warm-up prompts students to carefully analyze and compare expressions. In making comparisons, students have a reason to use language precisely. Listen for the language students use to describe and compare the expressions with a focus on descriptions of the digits, the operations, place value, and whether or not units may be composed or decomposed when using methods based on place value (MP6).

### Launch

- Groups of 2
- Display the image.

### Activity

- “Pick one that doesn’t belong. Be ready to share why it doesn’t belong.”
- 1 minute: quiet think time
- 2–3 minutes: partner discussion
- Record responses.

### Student Facing

Which one doesn’t belong?

- \(74 - 23\)
- \(24 + 37\)
- 4 tens + 2 ones + 3 tens + 7 ones
- \(60 + 19\)

### Student Response

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### Activity Synthesis

- “Let’s find one way each expression doesn’t belong.”

## Activity 1: Which Would You Rather Find? (20 minutes)

### Narrative

The purpose of this activity is for students to add and subtract within 1,000. Students begin the activity by analyzing different expressions without adding or subtracting to determine which values they think would be the least and most challenging to find. In the synthesis, students share different reasons why sums and differences were chosen to be “least” and “most challenging” and different methods for finding the values of the expressions.

Although students may have many different reasons for why they think expressions are more or less challenging to find, the synthesis focuses on reasons based on composing or decomposing units. Discussing whether students will compose or decompose units will support students in fluently finding sums and differences within 100 in the next activity.

This activity uses *MLR8 Discussion Supports. Advances: speaking*

### Launch

- Groups of 2

### Activity

- “Without finding the value of any of the sums or differences, read each expression and circle the two you think would be the most challenging to find the value of.”
- 3 minutes: independent work time
- “Choose one of the expressions you circled and find the value. Then choose 2 other expressions and find their value.”
- 4 minutes: independent work time
- “Share your work with your partner. Explain why you picked your expressions and the method you used to find the values.”

**MLR8 Discussion Supports**

- Display sentence frames to support students when they explain their strategy:
- “I picked _____ because . . . .“
- “First, I _____ because . . . .”
- “I noticed _____ so I . . . .”

- 4 minutes: partner discussion

### Student Facing

A

\(102 + 341\)

G

\(64 + 37\)

B

\(385 - 124\)

H

\(302 - 198\)

C

\(611 - 45\)

I

\(73 - 27\)

D

\(87 + 38\)

J

\(685 + 279\)

E

\(571 - 268\)

K

\(320 + 509\)

F

\(885 - 142\)

L

\(53 + 52\)

- Circle 2 sums or differences you think would be the most challenging to find. Find the value of one of these expressions. Show your thinking.
- Choose 2 other expressions and find the values. Show your thinking.

### Student Response

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### Activity Synthesis

- Poll the class. For each expression, ask students to “Show one finger if the expression was not challenging, two fingers if it was a little challenging, or three fingers if it was the most challenging to find.”
- Consider displaying:
- A, D, E, F, H, J

- Stop for any expressions where there is a mix of ratings.
- Consider asking:
- “Why did you feel this expression is more challenging?”
- “Who can share why this expression was less challenging?”

- For expressions where most or all students felt the expression was most challenging, select students to share different methods for finding the value.
- “All the expressions you felt were most challenging can become less challenging with practice and using what you know about place value.”

## Activity 2: Add and Subtract within 100 (15 minutes)

### Narrative

The purpose of this activity is for students to fluently find sums and differences within 100. Students should be encouraged to use whatever method makes the most sense to them, including mental strategies. In the synthesis, students reflect on which values were least and most challenging to find.

*Engagement: Provide Access by Recruiting Interest.*Provide choice and autonomy. Provide access to white boards and markers, or colored pencils and large paper for students to create a poster of their work. Encourage number lines or notation that show student thinking. If time allows, let students view each other’s strategies in solving the same problems. Focus on the differences in strategies, but the similarities in answers.

*Supports accessibility for: Attention, Organization, Social-Emotional Functioning*

### Launch

- Groups of 2

### Activity

- “Now let’s practice adding and subtracting with fluency.”
- “Find the value of these expressions. Use any method that makes sense to you. You do not need to show your work with a representation or symbols, but be ready to share how you found each value.”
- 5 minutes: independent work time
- “Share your thinking with your partner. Find 1 sum or difference that you both felt was the less challenging and 1 sum or difference that you both feel was more challenging.”
- 3 minutes: partner discussion
- Monitor for students who:
- describe why A, C, E, or G are less challenging because they do not need to compose or decompose
- describe why B, F, H, or J are more challenging because units are composed or decomposed or because they need to do more steps (in their head or on paper)
- describe why D, I, K, or L were less challenging because they saw ways to count up or back quickly, make a 10, or used facts they know

### Student Facing

- Find the value of each expression.
A

\(31 + 25\)G

\(95 - 81\)B

\(43 + 28\)H

\(37 + 55\)C

\(89 - 22\)I

\(25 + 25\)D

\(40 - 28\)J

\(63 - 19\)E

\(15 + 72\)K

\(72 - 68\)F

\(85 - 37\)L

\(21 + 59\) - Choose one value that you think was less challenging to find. Explain.
- Choose one value that you think was more challenging to find. Explain.

### Student Response

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### Advancing Student Thinking

- “What do you know about the numbers in this expression?”
- “What methods from previous lessons could you use to find the value?”

### Activity Synthesis

- Share 1–2 sums students felt were least challenging.
- Invite previously identified students to share methods they used for finding the sums or differences mentally.
- Share 1–2 sums that students felt were most challenging.
- Invite previously identified students to share different methods for finding the values.

## Lesson Synthesis

### Lesson Synthesis

“Today we practiced adding and subtracting with three-digit numbers. We also practiced fluently adding and subtracting two-digit numbers.”

“When did you notice it was easier to find the values of expressions?” (If I noticed I could just add or subtract the hundreds, tens, and ones without any extra steps.)

“What methods did you hear from other students today that you want to try when adding or subtracting?”

## Cool-down: Add and Subtract Within 1,000 (5 minutes)

### Cool-Down

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