Lesson 20

The purpose of this Number Talk is to elicit strategies and understandings students have for subtracting within 100. It also provides an opportunity to observe student strategies as they work toward becoming fluent in addition within 1,000.

When students use strategies based on place value to subtract, they look for and make use of structure (MP7).

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

• Record answers and strategy.
• Keep expressions and work displayed.
• Repeat with each expression.

• “How was place value helpful as you found the difference in these problems?” (I subtracted the tens first then adjusted my answer each time. I subtracted tens from tens and ones from ones.)
• Consider asking:
  • “Who can restate _____'s reasoning in a different way?”
  • “Did anyone have the same strategy but would explain it differently?”
  • “Did anyone approach the problem in a different way?”
  • “Does anyone want to add on to _____’s strategy?”

The purpose of this activity is to introduce the commutative property. Students write array situations for a pair of arrays and discuss similarities and differences. While the situations will have the same total number of objects, how the objects are grouped should be different. Then, students write equations to go with the arrays and situations, and make connections between the representations (MP2). Students notice that, while the order of the factors in the multiplication equation changes, the product does not change (MP7).

• Groups of 2
• Display the image.
• “What do you notice? What do you wonder?” (Students may notice: Both groups of dots are arranged as arrays. They both have 10 dots. One array has groups of 2. One array has groups of 5. Students may wonder: Why are the dots grouped differently? Are the arrays the same?)
• 1 minute: quiet think time
• “Discuss your thinking with your partner.”
• 1 minute: partner discussion
• Share and record responses.

• “Let’s consider these two arrays in more detail. Write an array situation for each array.”
• 3–5 minutes: independent work time
• “Share your situations with your partner. Together, consider how the situations you wrote for the first array are the same and different from the situations you wrote for the second array.”
• 3 minutes: partner work time
• “What was the same and what was different about the situations you wrote?”
• Share responses.
• “Now, write an equation for each situation you just came up with to match the arrays.”
• 1 minute: independent work time
• “Share your equations with your partner and discuss how each number in your equations connects to your situations and the array.”
• 2 minutes: partner discussion

• “Let’s write down the equations we came up with.”
• Display \(2\times5=10\) and \(5\times2=10\)
• “How do each of the numbers in the equations connect to the situation you wrote?” (5 is the number of columns in one situation, but it's how many dots are in each row in the other situation. 2 is the number of groups in one situation, but it's how many are in each group in the other situation. 10 is the total number of objects in both situations.)

The purpose of this activity is to reinforce the idea of the commutative property. In this activity, students write two equations to match an array to show again that reversing the order of the factors does not change the product. If students do not immediately see how they might write different equations for the array, encourage them to consider different ways of grouping the dots in the array, similar to the previous activity. Students use the vocabulary they have learned for describing arrays and multiplication to explain why both equations match an array with their partner. The Stronger and Clearer Each Time routine allows students to receive feedback and revise their explanation for clarity (MP3, MP6).

If students finish early, consider drawing another array. Have students write two equations for the array and consider how they can think of the rows or columns as equal groups.

This activity uses MLR1 Stronger and Clearer Each Time. Advances: reading, writing

• Groups of 2
• “Talk to your partner about equations that could represent this array.”
• 1 minute: partner discussion

• “Write two equations for this array. If it helps you, you can imagine grouping the dots as in the previous activity. Then write down why both equations can represent the array.”
• 5 minutes: independent work time

• “Let’s write down the equations we came up with.”
• Display the equations \(3\times6=18\) and \(6\times3=18\).

MLR1 Stronger and Clearer Each Time

• “Share why both equations can represent the array with your partner. Take turns being the speaker and the listener. If you are the speaker, share your ideas and writing so far. If you are the listener, ask questions and give feedback to help your partner improve their work.”
• 3–5 minutes: structured partner discussion.
• Repeat with 2–3 different partners.
• “Revise your initial draft based on the feedback you got from your partners.”
• 2–3 minutes: independent work time
• Have students share the revisions they made to their initial draft.