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
The purpose of this Number Talk is to elicit strategies and understandings students have for subtracting numbers within 1,000. These understandings help students develop fluency and will be helpful as students relate subtraction algorithms to strategies they have used to subtract within 1,000.
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.
\(50 - 10\)
\(58 - 10\)
\(258 - 20\)
\(258 - 24\)
Student Response
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Activity Synthesis
“How was place value helpful as you subtracted these numbers?” (When we were subtracting 10, only the tens place changed. For the last expression we were able to subtract the tens, then the ones.)
Consider asking:
“Who can restate _____'s reasoning in a different way?”
“Did anyone have the same strategy but would explain it differently?”
“Did anyone approach the problem in a different way?”
“Does anyone want to add on to _____’s strategy?”


aligns: no