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
The purpose of this Number Talk is to elicit strategies and understandings students have for adding within 20.
Students may have many ideas for adding these numbers. In the synthesis, students discuss what they notice about the first two expressions. Students may notice that the expressions are equal and that one expression might help them with the other.
Launch
Display one expression.
“Give me a signal when you have an answer and can explain how you got it.”
1 minute: quiet think time
Activity
Record answers and strategy.
Keep expressions and work displayed.
Repeat with each expression.
Student Facing
Find the value of each expression mentally.
\(8 + 2 + 4\)
\(8 + 6\)
\(7 + 3 + 6\)
\(7 + 9\)
Student Response
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Activity Synthesis
“What do you notice about the first two expressions?” (They are both 14. If 6 was broken into 2 and 4, it would be the same expression.)


aligns: no