standard_description: Prove that, given a system of two equations in two variables, replacing one equation by the sum of that equation and a multiple of the other produces a system with the same solutions.
problem_activity: Narrative
In this activity, students analyze addition and subtraction calculations, identify errors, and explain what makes certain ways of calculating problematic. The work here gives students opportunities to construct logical arguments and critique the reasoning of others (MP3).
MLR8 Discussion Supports.
During group work, invite students to take turns sharing their responses. Ask students to restate what they heard using precise mathematical language and their own words. Display the sentence frame: “I heard you say . . . .” Original speakers can agree or clarify for their partner.
Advances: Listening, Speaking
Required Materials
Materials to Gather
Grid paper
Launch
Groups of 2
Display the first problem.
2 minutes: quiet think time
2–3 minutes: partner discussion
Activity
5 minutes: independent work time to complete the second problem.
2 minutes: partner discussion
Monitor for students who describe the errors using place value language and an understanding of how and when to decompose a unit.
Student Facing
Kiran is trying to find the sum of 204,500 and 695. He isn’t sure how to set up the calculation so he wrote down two ideas. Which way is correct? Be ready to share your thinking with your partner.
A
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B
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Lin made some errors when subtracting 4,325 from 61,870. Identify as many errors as you can find. Then, show the correct way to subtract.
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Student Response
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Advancing Student Thinking
If students have trouble making sense of Lin's work, encourage them to find the difference and then compare their work to Lin's.
Activity Synthesis
Select 2–3 students to share their responses and reasoning.
Record responses as students share.
“What are some errors that are commonly done when subtracting two numbers using the standard algorithm?” (Subtracting without lining up the numbers by place value. Using the notation of regrouping without decomposing a unit. Crossing out digits without including them in the regrouping. Subtracting from left to right without decomposing units.)
“What ideas do you have for avoiding those errors?” (Use grid paper to line up the digits by place value. Decompose the unit before regrouping. Check answers using addition.)


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