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 True or False is to elicit strategies students have for reasoning about place value to determine if an equation is true or false. The reasoning students do here helps to deepen their understanding of using strategies based on place value. It will also be helpful later when students reason about how to reach 100 from a given number using the number line.
Launch
Display one statement.
“Give me a signal when you know whether the statement is true and can explain how you know.”
1 minute: quiet think time
Activity
Share and record answers and strategy.
Repeat with each statement.
Student Facing
Decide if each statement is true or false. Be prepared to explain your reasoning.
\(28 + 2 + 20 = 50\)
\(16 + 3 + 30 = 50\)
\(36 + 4 + 20 = 50\)
Student Response
Teachers with a valid work email address can
click here to register or sign in
for free access to Student Response.
Activity Synthesis
“How can you explain your answer to the last statement without finding the sum of all three addends?” (I could add 36 and 20 and see that it’s more than 56. I could add 36 and 4 to get 40. I know I would need 10 more to get to 50 and I see more than 10 left to add.)


aligns: no