Lesson 26
What’s the Story?
Warmup: Number Talk: Subtract 10 or More (10 minutes)
Narrative
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 \(10 + n\) 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 strategy.
 Keep expressions and work displayed.
 Repeat with each expression.
Student Facing
Find the value of each expression mentally.
 \(15  10\)
 \(15  12\)
 \(16  10\)
 \(16  13\)
Student Response
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Activity Synthesis
 “How can \(15  10\) help with \(15  12\)?” (We know 12 is 10 and 2 so we can use \(15  10\) and subtract 2 more.)
Activity 1: Solve Related Story Problems (15 minutes)
Narrative
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.
Students write equations to represent each problem and there are many equations students 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 \( 6 + \boxed{\phantom{3}} = 18\) students may write:
 \(6 + 4 = 10\)
 \(10 + 8 =18\)
 \(4 + 8 = \boxed{12}\)
Advances: Reading, Representing
Required Materials
Materials to Gather
Launch
 Groups of 2
 Give students access to double 10frames and connecting cubes or twocolor 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:
 \(6 + \boxed{12} = 18\)
 \(18  6 = \boxed{12}\)
 \(18  \boxed{12} = 6\)
Student Facing

Elena has 6 counters.
She gets some more counters.
Now she has 18 counters.
How many more counters did Elena get?
Show your thinking using drawings, numbers, or words.Equation: ________________________________

Elena has 18 counters.
She gets rid of some counters.
Now she has 6 counters.
How many counters did Elena get rid of?
Show your thinking using drawings, numbers, or words.Equation: ________________________________
Student Response
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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 \(6 + 12\) or subtract \(18  6\) for both problems to solve it.)
 “Why is the missing number the same in each of these equations?” (Because \(6 + 12 = 18\) and \(18  6 = 12\). The difference is the same whether I add or subtract.)
Activity 2: More Story Problems (25 minutes)
Narrative
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.
Supports accessibility for: Conceptual Processing
Required Materials
Launch
 Groups of 2
 Give each group tools to create a visual display and access to double 10frames and connecting cubes or twocolor counters.
 Assign each group a story problem to solve.
 “Work with your partner to solve the story problem and create a poster showing how you solved. 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
Student Facing
Story Problem 1
Han has some pencils.
He gets 9 pencils from the art store.
Now he has 15 pencils.
How many pencils did Han have to start?
Story Problem 2
Han has 15 pencils.
He gives some pencils to his 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 pencils from the art store.
Now he has 15 pencils.
How many pencils did he get from the art store?
Story Problem 4
Han has 15 pencils.
He gives 9 pencils to his friends.
How many pencils does Han have now?
Show your thinking using drawings, words or numbers.
Equation: _________________________________
Student Response
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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
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.)
Cooldown: Unit 3, Section D Checkpoint (0 minutes)
CoolDown
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Student Section Summary
Student Facing
We used different methods to subtract within 20.
We used take away methods.
\(15  8\)
We used a ten to take away 8.
We used counting on methods.
\(15  8\)
8. . . 9, 10, 11, 12, 13, 14, 15
Use ten to help count on.
\( 8 + 2 = 10\)
\(10 + 5 = 15\)
\(2 + 5 = 7\)