standard_description: Mentally add 10 or 100 to a given number 100–900, and mentally subtract 10 or 100 from a given number 100–900.
problem_activity: Narrative
In the second round of analysis and design, students use their knowledge of operations and expressions to complete a Which One Doesn’t Belong set with two missing items. Students first study the attributes of the given items. They articulate why the items all belong to a set and why each one has a reason not to belong. Next, they propose a new item that shares a feature with others in the set but also has a unique feature that excludes it from the set.
Launch
Groups of 2
5 minutes: independent work time
Activity
“Work with your partner to make sure each new item added to the set has a reason to belong and not to belong.”
5 minutes: partner work time
Student Facing
Here is an incomplete Which One Doesn’t Belong set. It has two missing items.
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Find at least one reason that the first two items, A and B, belong in the set.
Add two items to complete the set. Make sure there is at least one reason that each new item belongs and at least one reason it doesn’t belong.
C and D both belong because . . .
C doesn't belong because . . .
D doesn't belong because . . .
After you’ve completed the set, check items A and B. Does each one still have a reason not to belong? If not, adjust your new items so that A and B are each still unique in some way.
Student Response
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Activity Synthesis
Invite students to share their analyses and new items.
“Why do both A and B belong to the set?”
“What were some features of the expressions that you looked at or experimented with to complete your set?” (The kind of numbers used, how many numbers are used, the order of the numbers, the symbols, the value of the expression.)
If no students considered the value of the expression, ask them about it.
“After adding C and D, did A and B still have a reason not to belong in your set? Did you have to revise your items?”


aligns: no