# Lesson 14

Think Before You Subtract

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

### Narrative

This warm-up prompts students to carefully analyze and compare base-ten diagrams. The activity also enables the teacher to hear the terminologies students know and how they talk about composing and decomposing numbers with hundreds, tens, and ones.

### 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

• 2–3 minutes: partner discussion
• Share and record responses.

### Student Facing

Which one doesn’t belong?

### Activity Synthesis

• “What do A, B, and C have in common?” (They all show 125.)
• “Why would you want to represent 125 with 12 tens instead of 1 hundred and 2 tens?” (If you are going to subtract more than 2 tens.)

## Activity 1: Agree to Disagree (15 minutes)

### Narrative

The purpose of this activity is for students to use their understanding that numbers can be decomposed in different ways to subtract within 1,000. They use what they know about place value to make sense of two different ways for finding the same difference (MP7). They describe how it is helpful to represent the number with enough tens and ones to subtract, rather than needing to redraw or exchange blocks to represent decomposing a unit. They also learn that sometimes it may be necessary to decompose a hundred and a ten to subtract within 1,000.

### Required Materials

Materials to Gather

### Launch

• Groups of 2
• “Tyler and Clare each used base-ten diagrams to find the value of $$244-67$$. Tyler and Clare both agreed that they should decompose units before they subtract.”
• “Look at Tyler and Clare’s first steps. What did they do? What do you believe they were they thinking?”
• 1–2 minutes: quiet think time
• 2 minutes: partner discussion
• Share responses.

### Activity

• “Work with your partner to complete Tyler’s way and Clare’s way. When you finish, compare the diagrams.”
• 4 minutes: partner work time

### Student Facing

Tyler and Clare are subtracting by place to find the value of $$244-67$$. Tyler says he will decompose before he starts. Clare says she agrees.

The diagrams show each student’s first step.

Tyler:

Clare:

1. What is the same about Tyler and Clare’s diagrams? What is different?
2. Work together to complete Tyler’s way and Clare’s way to find the value of $$244-67$$.
3. What do Tyler and Clare’s diagrams look like after the last step? What is the same about these diagrams? What is different?

### Student Response

If students do not see that Tyler’s first step and Clare’s first step show the same number, prompt students to use base-ten blocks or a base-ten diagram to show 244. Consider asking:
• “If your representation shows 244, what unit did Tyler decompose?”
• “If your representation shows 244, what unit did Clare decompose?”

### Activity Synthesis

• Display student work that shows using Tyler’s way and Clare’s way to find the difference.
• “How were Tyler’s way and Clare’s way the same? How were their methods different?” (They both show decomposing a ten and a hundred to subtract. They both show the same difference. Tyler decomposed a hundred first, then a ten. Clare decomposed a ten first, then a hundred.)

## Activity 2: Sort and Subtract (20 minutes)

### Narrative

The purpose of this activity is for students to think about the numbers and attend to the value of the digits in subtraction expressions before beginning to subtract (MP7). Students determine whether or not they will decompose to subtract by place. They also determine whether they will decompose more than one unit. Students find the value of different expressions by using any method that makes sense to them.

Action and Expression: Internalize Executive Functions. Check for understanding by inviting students to rephrase directions in their own words. Be sure students can explain when it is necessary to decompose.
Supports accessibility for: Memory, Organization

### Required Materials

Materials to Gather

### Launch

• Groups of 2
• Draw or display the base-ten diagram for 341.
• “Andre wants to use a diagram to find the value of $$341-68$$. He says he will decompose a ten and a hundred to subtract. Why do you think he said that?” (If you take tens from tens, there’s not enough to take 6 tens from 4 tens. If you take ones from ones, there are not enough ones to take 8 ones from 1 one.)
• 1 minute: quiet think time
• 1 minute: partner discussion
• “If Andre knows he will decompose a hundred and a ten, what’s another way he could have started his diagram?” (He could have started with 3 hundreds, 3 tens, and 11 ones. He could have started with 2 hundreds, 14 tens, and 1 one. He could have started with 2 hundreds, 13 tens, and 11 ones.)
• 1 minute: quiet think time
• 1 minute: partner discussion
• Share and record responses.

### Activity

• “Andre only wants to use a diagram to subtract by place if he will decompose a unit.”
• “Work with your partner to sort Andre's expressions into 3 groups. Look at the numbers in each expression and determine if you would decompose 2 units, 1 unit, or 0 units to subtract by place. Explain to your partner how you know. Use base-ten blocks or create your own diagrams to help. Then, write the expression in the appropriate column.”
MLR8 Discussion Supports
• Display sentence frames to support students when they explain their strategy and listen to others:
• “I noticed ____ so I think that . . . ”
• “I heard you say . . .”
• “I agree because . . .”
• “I disagree because . . .”
• 8 minutes: partner work time

### Student Facing

Here is a base-ten diagram for 341.

Andre wants to use diagrams and subtract by place to find the value of $$341 - 68$$. He says he will decompose a ten and a hundred to subtract. Why do you think he said that?

1. Andre only wants to use a diagram to subtract by place if he will decompose a unit. Help Andre sort the expressions into groups. If you are not sure, use base-ten blocks or a diagram to help.

$$599 - 66$$

$$449 - 88$$

$$346 - 78$$

$$633 - 55$$

$$237 - 29$$

$$321 - 34$$

$$457 - 45$$

$$735 - 72$$

$$645 - 87$$

$$905 - 42$$

$$693 - 63$$

$$866 - 58$$

$$514 - 26$$

$$387 - 44$$

$$277 - 65$$

decompose 2 units decompose 1 unit do not decompose

2. Find the value of 1 expression from each group. Show your thinking.

### Activity Synthesis

• Display a completed chart from student responses.
• Ask students to choose an expression and explain how they know it belongs in that column.
• “When you were finding the difference, did you use the same method for all 3 expressions? How did you choose your method?” (I used the blocks when I needed to decompose, but I didn’t need to when I didn’t decompose so I just wrote equations. Since I knew before starting when I was going to decompose, I knew I didn’t need the blocks for one of them,)

## Lesson Synthesis

### Lesson Synthesis

“Today we looked at expressions and thought about how we could decompose units to subtract by place. When you subtract by place, why is it helpful to think about where you may need to decompose a unit before beginning to find the difference?” (It might help you make sense of the expression before you subtract. It can help you draw a diagram with fewer steps, you can start with some of the units already decomposed.)