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 the ways students look for to use the structure of two-digit numbers to subtract (MP7). These understandings help students develop fluency and will be helpful later in this lesson when students will need to be able to find the difference between the estimate and actual measurement of an object.
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.
\(37 - 20\)
\(37 - 21\)
\(37 - 17\)
\(37 - 16\)
Student Response
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Activity Synthesis
“How could you use
\(37-20\)
to help you with
\(37 - 21\)
?”


aligns: no