standard_description: Given a two-digit number, mentally find 10 more or 10 less than the number, without having to count; explain the reasoning used.
problem_activity: Narrative
This warm-up prompts students to compare four comparison statements. It gives the teacher an opportunity to hear how students use terminology and talk about characteristics of the items in comparison. During the synthesis, ask students to explain the meaning of any terminology they use, such as comparing, greater than, and less than.
Launch
Groups of 2
Display the image.
“Pick one that doesn’t belong. Be ready to share why it doesn’t belong.”
1 minute: quiet think time
Activity
“Discuss your thinking with your partner.”
2–3 minutes: partner discussion
Share and record responses.
Student Facing
Which one doesn’t belong?
A
[IMG]
B
[IMG]
C
[IMG]
D
[IMG]
Student Response
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Activity Synthesis
“How do you know that C is false?” (35 isn't less than 20 because 35 has 3 tens and 20 only has 2 tens.)
“What could you change about C to make it true?” (Use the greater than symbol or switch the symbol around.)


aligns: no