standard_description: Understand subtraction as an unknown-addend problem. For example, subtract 10 – 8 by finding the number that makes 10 when added to 8.
problem_activity: Narrative
In this activity, students examine the rectangles drawn by their classmates and learn the term
factor pairs
. Students recognize the side lengths of each rectangle as a factor pair of its area.
This activity uses
MLR7 Compare and Connect
.
Advances: representing, conversing
Launch
Groups of 2
Activity
5–7 minutes: gallery walk
Monitor for different explanations students offer for how they know whether all possible rectangles with a given area have been found.
Student Facing
As you visit each poster, discuss with your partner:
What do you notice? Use the following sentence frames when you share:
“I notice that some of the posters . . . .”
“I notice the posters for numbers _____ and _____ are alike because . . . .”
How do you know that all possible rectangles were found for the given area?
Student Response
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Activity Synthesis
MLR7 Compare and Connect
“What is the same and what is different between the rectangles on the posters?”
30 seconds: quiet think time
1 minute: partner discussion
“How do you know that all possible rectangles have been found for the given area?” (We could not find any other numbers that multiply together to make the area.)
Display the rectangles for 21: 1 by 21 and 3 by 7.
“Are there any more rectangles we can draw? Why or why not?” (No, there are no more whole-number factors of 21. Or, no, because to get 21 we can multiply only 1 and 21, 3 and 7, 7 and 3, and 21 and 1.)
“We call 1 and 21 a
factor pair
of 21 because each of them is a factor of 21 and multiplying them gives 21. Another factor pair of 21 is 3 and 7.”
“Work with your partner to write down the factor pairs for the areas you were assigned.”
2 minutes: partner work time


aligns: no