standard_description: Fluently add and subtract within 20 using mental strategies. (See standard 1.OA.6 for a list of mental strategies.) By end of Grade 2, know from memory all sums of two one-digit numbers.
problem_activity: Narrative
The purpose of this activity is for students to use their understanding of place value to find the number that makes each equation true. Students must consider how units may be composed or decomposed to find the unknown number (MP7). The number choices intentionally emphasize the types of compositions and decompositions students may use to add and subtract within 1,000. The synthesis focuses on the equations that involve composing or decomposing two units.
Required Materials
Materials to Gather
Base-ten blocks
Launch
Groups of 2
Give students access to base-ten blocks.
Activity
5 minutes: independent work time
“Take turns sharing your thinking with your partner. Show or explain how you know your equation is true.”
3 minutes: partner discussion
For finding the number that makes 2 hundreds + 9 tens + 17 ones true, monitor for students who:
explain by using base-ten blocks or place value diagrams
explain how they noticed when they would need to compose or decompose more than one unit
Student Facing
Find the number that makes each equation true.
5 hundreds + _____ tens + 6 ones = 5 hundreds + 5 tens + 16 ones
1 hundred + 1 ten + 17 ones = 1 hundred + _____ tens + 7 ones
4 hundreds + 15 tens + 3 ones = _____ hundreds + 5 tens + 3 ones
4 hundreds + 3 tens + 7 ones = 3 hundreds + _____ tens + 7 ones
7 hundreds + 8 tens + 4 ones = 7 hundreds + 7 tens + _____ ones
[IMG]
6 hundreds + 9 ones = 5 hundreds + _____ tens + 9 ones
2 hundreds + 9 tens + 17 ones = _____ hundreds + 7 ones
3 hundreds + 1 ten + 5 ones = 2 hundreds + 10 tens + _____ ones
If You Have Time: Make your own equation with a missing value for your partner.
Student Response
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Advancing Student Thinking
If students create equations that are not true, prompt students to make the known side of the equation with base-ten blocks, consider asking:
“How can you compose or decompose units to show the equation is true?”
Activity Synthesis
Invite previously identified students to share their method for finding the unknown number for 2 hundreds + 9 tens + 17 ones = _____ hundreds + 7 ones.
For students who use base-ten blocks, draw place value diagrams to record their explanation.
Consider asking:
“How do you know both sides of the equation are equal?”
“Did you compose or decompose units? Did you compose or decompose more than one time?”


aligns: no