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
The purpose of this activity is for students to choose from activities that offer practice adding two-digit numbers within 100. Students choose from any stage of previously introduced centers.
How Close?
Target Numbers
Five in a Row
Required Materials
Materials to Gather
Materials from previous centers
Required Preparation
Gather materials from:
How Close? Stages 1–3
Target Numbers, Stages 1–3
Five in a Row, Stages 1–6
Launch
Groups of 2
“Now you are going to choose from centers we have already learned.”
Display the center choices in the student book.
“Think about what you would like to do.”
30 seconds: quiet think time
Activity
Invite students to work at the center of their choice.
10 minutes: center work time
Student Facing
Choose a center.
How Close?
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Target Numbers
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Five in a Row
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Activity Synthesis
“Diego and Elena are playing How Close. Diego has a sum of 91. Elena has a sum of 89. Who gets a point for being closer to 100? How do you know?”


aligns: no