The purpose of this Number Talk is to elicit strategies and understandings students have for subtracting a teen number from another teen number. The expressions are sequenced to encourage students to break the subtrahend into a ten and some ones. Students can then subtract the ten and ones in two different steps. Based on the previous lesson, students may decompose the subtrahend into and subtract 10 first and then n.
Launch
Display one expression.
“Give me a signal when you have an answer and can explain how you got it.”
1 minute: quiet think time
Activity
Record answers and strategies.
Keep expressions and work displayed.
Repeat with each expression.
Find the value of each expression mentally.
Student Response
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Advancing Student Thinking
Activity Synthesis
“How can help you find the value of ?” (I know 12 is 10 and 2, so I can use and subtract 2 more.)
Activity 1
Standards Alignment
Building On
Addressing
1.OA.A.1
Use addition and subtraction within 20 to solve word problems involving situations of adding to, taking from, putting together, taking apart, and comparing, with unknowns in all positions, e.g., by using objects, drawings, and equations with a symbol for the unknown number to represent the problem.See Glossary, Table 1.
The purpose of this activity is for students to solve two story problems that highlight the relationship between addition and subtraction. Both are Change Unknown stories that use the same numbers. Although one story “sounds” like addition and the other subtraction, both stories can be solved using either operation. The same equations can be used to solve both problems.
If students choose to write equations to represent each problem, there are many equations they could write. The important thing is for students to be able to explain how the equation they wrote matches the story problem. Some students may write each of their steps as equations.
For example, for students may write:
MLR6 Three Reads. Keep books or devices closed. To launch this activity, display only the problem stem, without revealing the question. “We are going to read this story problem three times.”
After the 1st Read: “Tell your partner what happened in the story.”
After the 2nd Read: “What are all the things we can count in this story?” Reveal the question.
After the 3rd Read: “What are different ways we can solve this problem?”
Advances: Reading, Representing
Launch
Groups of 2
Give students access to double 10-frames and connecting cubes or two-color counters.
Activity
Read the Task Statement.
5 minutes: independent work time
3 minutes: partner discussion
Monitor for students who write and can explain a variety of equations such as:
Activity Synthesis
Invite previously identified students to share their equation for each problem.
If needed, ask, “How does your equation match how you solved the problem?”
“What do you notice about the equation used to solve each problem?” (They could be the same. You can add or subtract for both problems to solve it.)
“Why is the unknown number the same in each of these equations?” (Because and . The difference is the same whether I add or subtract.)
Activity 2
Standards Alignment
Building On
Addressing
1.OA.A.1
Use addition and subtraction within 20 to solve word problems involving situations of adding to, taking from, putting together, taking apart, and comparing, with unknowns in all positions, e.g., by using objects, drawings, and equations with a symbol for the unknown number to represent the problem.See Glossary, Table 1.
The purpose of this activity is for students to solve related addition and subtraction story problems with the unknown in different positions. Students work with a partner to solve a story problem and create a poster of their work. They share their work with groups who solved a different problem and compare their representations and methods.
Representation: Access for Perception. Invite students to act out the scenario of their assigned story problem before solving. Supports accessibility for: Conceptual Processing
Launch
Groups of 2
Give each group tools to create a visual display and access to double 10-frames and connecting cubes or two-color counters.
Assign each group a story problem to solve.
Read the problems as a class.
“Work with your partner to solve the story problem and create a poster showing how you solved the problem. Be sure to include any equations you used. If you can solve the problem in more than one way, show the different ways and equations.”
Activity
8 minutes: partner work time
Arrange groups together so each larger group has students who have solved each of the four problems.
“Share your poster with your group. Explain how the equations you wrote match the story. As each group shares, discuss how the problems are the same and different. Make a list of equations you used for each problem.”
10 minutes: group work time
Story Problem 1
Han has some pencils.
He gets 9 more.
Now he has 15 pencils.
How many pencils did Han have to start?
Story Problem 2
Han has 15 pencils.
He gives some to friends.
Now he has 9 pencils.
How many pencils did Han give to his friends?
Story Problem 3
Han has 9 pencils.
He gets some more from the store.
Now he has 15 pencils.
How many pencils did Han get from the store?
Story Problem 4
Han has 15 pencils.
He gives 9 to friends.
How many pencils does Han have now?
Show your thinking using drawings, words or numbers.
Student Response
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Advancing Student Thinking
Activity Synthesis
Display a list of equations one group made for each problem.
“What do you notice about the equations this group used for each problem?” (They used the same equations for each problem. All the problems have more than one equation. All the problems have addition and subtraction equations.)
Lesson Synthesis
“We have been doing a lot of subtraction using different methods. Tell your partner something new you have learned about subtraction.” (I learned that you can turn a subtraction expression into an addition expression. I learned that you can use 10 to help you subtract.)
Student Section Summary
We used different methods to subtract within 20.
We used take away methods.
We used a ten to take away 8.
We used counting on methods.
8. . . 9, 10, 11, 12, 13, 14, 15
We used a ten to count on.
Standards Alignment
Building On
Addressing
1.OA.C.6
Add and subtract within 20, demonstrating fluency for addition and subtraction within 10. Use strategies such as counting on; making ten (e.g., ); decomposing a number leading to a ten (e.g., ); using the relationship between addition and subtraction (e.g., knowing that , one knows ); and creating equivalent but easier or known sums (e.g., adding by creating the known equivalent ).
Show your thinking using drawings, numbers, or words.
Elena has 6 counters.
She gets some more.
Now she has 18 counters.
How many more counters did Elena get?
Elena has 18 counters.
She gets rid of some.
Now she has 6 counters.
How many counters did Elena get rid of?
Student Response
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Advancing Student Thinking
1.OA.C.6
Add and subtract within 20, demonstrating fluency for addition and subtraction within 10. Use strategies such as counting on; making ten (e.g., ); decomposing a number leading to a ten (e.g., ); using the relationship between addition and subtraction (e.g., knowing that , one knows ); and creating equivalent but easier or known sums (e.g., adding by creating the known equivalent ).
Add and subtract within 20, demonstrating fluency for addition and subtraction within 10. Use strategies such as counting on; making ten (e.g., ); decomposing a number leading to a ten (e.g., ); using the relationship between addition and subtraction (e.g., knowing that , one knows ); and creating equivalent but easier or known sums (e.g., adding by creating the known equivalent ).
