Lesson 11

This Number Talk encourages students to think about decomposing the subtrahend to get to a ten when subtracting. For example, in the first problem it is helpful to think about 6 as 2 + 4. This way you can subtract 2 to get to 30, and then subtract 4 from 30. The understandings elicited here will be helpful later in the lesson when students represent sums and differences on a number line by jumping to the nearest ten.

In reasoning together about the number line representation, and connecting the strategy of making a ten to jumping to the nearest ten, students need to be precise in their word choice and use of language (MP6).

• 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 on a number line.
• Keep expressions and work displayed.
• Repeat with each expression.

• “For \(52 - 7\) some students decomposed the 7 to make it easier to get to a ten. How does this number line representation connect to that strategy?”

• Draw a number line showing 52 represented with a point, a jump of 2, and then a jump of 5.

The purpose of this activity is for students to use a number line to compare different methods for getting to a ten when subtracting a two-digit number from a two-digit number. Students analyze and try a method where they add or subtract the tens first and then decompose the ones to reach a multiple of 10. Diego's method is an example of this for \(53 - 29\).

They analyze a method that begins by decomposing the ones being added or subtracted to get to a multiple of ten. In the synthesis, students discuss each method and which one they prefer. Tyler’s method is an example of this for \(53 - 29\).

• Groups of 2
• Give each student a copy of the blackline master.
• “Diego and Tyler found the value of \(53 -29\) on the number line. Each student represented how they found the difference. Explain each student’s method.”
• 1 minute: quiet think time
• 2 minutes: partner discussion
• Share responses.
• “What is the same and what is different about how Diego and Tyler found the difference?” (Diego started by subtracting the tens and Tyler started by subtracting the ones. They both decomposed the ones to make subtracting the ones easier. They both thought of a way they could decompose to get to a ten.)

• “Now, you will have the opportunity to try out Diego and Tyler’s methods. Find the value of each expression. Represent the methods on the number line.”
• 8 minutes: independent work time
• 2 minutes: partner discussion

• Invite students to share how they used each Tyler and Diego's methods.
• “Did you find Tyler’s method or Diego’s method more helpful? Explain.”
• “Is there another way we could use the number line to show a way to make it easier to find the value of \(53-29\) by getting to a ten?”
• If not suggested by students, ask: “How could we start with 29 and use a method like Diego or Tyler’s?” (Start with 29, jump over 1 to get to 30, then it's easy to see all you need to do is jump 23 to get to 53.)
• Record the method and display throughout the lesson for students to reference.
• “How is this method the same as Diego or Tyler’s? How is different?”

The purpose of this activity is for students to continue to develop fluency with addition and subtraction within 100. The numbers in each expression encourage the use of the methods students analyzed in the previous activity. However, students should be encouraged to use whatever method makes the most sense to them. Partners work together to create a visual display to share their representations of 1 sum and 1 difference including number lines and do a gallery walk to compare their representations to others (MP2).

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

• Groups of 2
• Give each student a copy of the blackline master and access to base-ten blocks.
• Assign Partner A and Partner B.

• “Find the value of the sum and difference. You may continue to try Diego or Tyler's method or use any other way that makes sense to you. Use the number line if it helps to show your thinking.”
• 5 minutes: independent work time
• 3 minutes: partner discussion

• ”How did you decompose or compose a ten with the blocks? How can you tell by looking at the expression that you would need to decompose or compose a ten?”
• ”How is this like how your partner used the number line? How is it different?”

• “Create a visual display that shows your thinking for 1 sum and 1 difference with a partner.”
• “You should include a number line for at least one expression in your display. You may want to include base-ten diagrams or other details to help others understand your thinking.”
• 5 minutes: partner work time
• 6 minutes: gallery walk 
• ”What is the same and what is different between the representations?”