standard_description: Add within 100, including adding a two-digit number and a one-digit number, and adding a two-digit number and a multiple of 10, using concrete models or drawings and strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; relate the strategy to a written method and explain the reasoning used. Understand that in adding two-digit numbers, one adds tens and tens, ones and ones; and sometimes it is necessary to compose a ten.
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
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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