Lesson 7
Subtract Two Digits
Warmup: How Many Do You See: Compose a Ten (10 minutes)
Narrative
The purpose of this How Many Do You See is to build on what students know about place value to make sense of visual representations of twodigit numbers. When students describe how many they see by grouping tens with tens and ones with ones or composing a ten, they show how they look for and make use of baseten structure (MP7). This will be helpful when students use baseten representations to compose and decompose a ten during the lesson.
Launch
 Groups of 2
 “How many do you see? How do you see them?”
 Display the image.
 30 seconds: quiet think time
Activity
 Display the image.
 “Discuss your thinking with your partner.”
 1 minute: partner discussion
 Record responses.
 Repeat for each image.
Student Facing
How many do you see? How do you see them?
Student Response
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Activity Synthesis
 “Which two images have the same value? How do you know?”(The first image and the second image both have the same amount. The first one shows 5 tens and 5 ones. I know Image 2 is the same because I know 2 groups of 5 is the same as 1 ten. So it's the same as 5 tens and 5 ones.)
Activity 1: What's the Difference? (15 minutes)
Narrative
The purpose of this activity is for students to subtract a twodigit number from a twodigit number in a way that makes sense to them. Students build on their understanding of decomposing a ten when subtracting a onedigit number from a twodigit number to subtract twodigit numbers. The synthesis is devoted to presenting and comparing techniques students use to find the difference, including diagrams and equations (MP3).
This activity uses MLR8 Discussion Supports. Advances: listening, speaking, conversing
Supports accessibility for: VisualSpatial Processing, Language, Memory
Required Materials
Materials to Gather
Launch
 Groups of 2
 Give students access to connecting cubes and baseten blocks.
Activity
 “Find the value of each difference and share your method and solution with your partner.”
 7 minutes: independent work time
MLR8 Discussion Supports
 “After your partner shares their method, repeat back what they told you.”
 Display the sentence frames:
 “I heard you say . . . .”
 “Our methods are alike because . . . .”
 “Our methods are different because . . . .”
 5 minutes: partner discussion
 Monitor for students who use baseten blocks to show decomposing a ten.
Student Facing
Find the value of each difference. Show your thinking. Use blocks if it helps.
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Student Response
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Advancing Student Thinking
 “How did you find the value of the difference?”
 “What could you add to your diagram to help your partner understand what you did?”
 “How could you use equations to show the steps you used?”
Activity Synthesis
 Invite previously identified students to share.
 “What did _____ do to solve the problem? How can we record it?”
 30 seconds: quiet think time
 Share responses.
 Show how to represent solving with baseten blocks with drawings and equations.
 “How are these representations like the way _____ used baseten blocks to find the value of the difference? How are they different?”
Activity 2: Use Blocks to Take Away (20 minutes)
Narrative
The purpose of this activity is for students to subtract a twodigit number from a twodigit number. In the first activity, students used any method that made sense to them to find the difference. In this activity, they use baseten blocks to represent the starting number and subtract amounts that require a ten to be decomposed.
Required Materials
Required Preparation
 Create a set of cards from the blackline master for each group of 4.
Launch
 Groups of 4
 Give each group a set of cards and access to baseten blocks.
Activity
 “Now you are going to play a card game using baseten blocks.”
 “First, each member of your group will choose a different player card, Diego, Lin, Jada, or Han.”
 “Write down the name you picked. Mix up the other cards and put them face down.”
 “Now you each have a player and your starting number.”
 “Use the blocks to represent your starting numbers.”
 2 minutes: group work
 Guide students through the rest of the steps:
 “Take turns picking a card and read the card to the group.”
 “Listen for your player’s name and follow the directions on the card.”
 “Share your thinking while your group members listen. Write an equation to show the new number.”
 “Before picking a new card, make sure your group works together to agree on the new number.”
 “You will keep playing until all of the cards have been read. Your player‘s number should change two times.”
 15 minutes: group work time
 Monitor for students who:
 discuss decomposing a ten using precise language
 work together to give feedback and resolve any disagreements
 compose tens when adding the ending numbers
Student Facing

Choose a player card. Mix up the other cards and put them face down.
Player name: ________________________________

Represent your starting number with baseten blocks.
Starting number: ____________________________
 Take turns picking a card. Read the card to the group.
 Listen for your player’s name. Use the blocks to show the change.
 Explain your thinking to your group.

Write an equation to show the new number.
Equation 1: ________________________________
Equation 2: ________________________________
My player now has ____________ tens and ____________ones.
Ending number: _____________________________
Share this number with your group.

Write an equation to show the sum of the ending numbers in your group.
Student Response
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Activity Synthesis
 Invite 1–2 previously identified students to share with their group.
 Consider asking:
 "What questions do you have for __ about their method?"
 "How was their method the same as how you found your player's new numbers? How was it different?"
Lesson Synthesis
Lesson Synthesis
“Today you used different methods to subtract 2 twodigit numbers.”
“You solved in ways that make sense to you and you used baseten blocks to take away different amounts.”
“How was subtracting a twodigit number from a twodigit number the same as subtracting a onedigit number from a twodigit number?" (It was the same because we still had to think about subtracting ones. Sometimes you have to decompose a ten if you need more ones.)
"How was it different?" (When you subtract a twodigit number, you have to think about subtracting tens too. The number you are subtracting is bigger because it's a twodigit number.)
"How did working together with a partner or a group help you understand different ways to subtract twodigit numbers?"
Math Community
“The card game required you all to work together. What are some ways your group worked together?” (We helped each other with keeping track of the blocks. We talked about our thinking. We helped each other if someone was stuck or confused.)
Cooldown: Decompose to Subtract (5 minutes)
CoolDown
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