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 subtract a one-digit number from a two-digit number. In the previous activity, students shared many ways to subtract including using connecting cubes or base-ten blocks to show decomposing a ten. They build on this understanding as they use base-ten blocks to represent the starting number and subtract an amount that requires them to decompose a ten.
MLR8 Discussion Supports.
Synthesis: Display sentence frames to support students with preparing to explain their thinking in the whole-class discussion. “First, I _____ because . . . .” “I noticed _____ so I . . . .” If necessary, revoice student ideas to demonstrate mathematical language, and invite students to chorally repeat phrases that include relevant vocabulary in context.
Advances: Speaking, Conversing
Representation: Internalize Comprehension.
Synthesis: Invite students to identify which details they think are important to remember. Use the sentence frame: “The next time I subtract, I will know that I need to decompose a ten when . . . .“
Supports accessibility for: Conceptual Processing, Memory
Required Materials
Materials to Gather
Base-ten blocks
Launch
Groups of 2
Give each group access to base-ten blocks.
Activity
“Diego was representing numbers using base-ten blocks. Work with a partner to follow along and see what Diego discovered.”
“Use your blocks first to show what Diego does. Then answer any questions.”
8 minutes: partner work time
Monitor for students who talk about “exchanging” or “trading” a ten for ten ones.
Student Facing
Diego started with 5 tens and 5 ones. Represent Diego’s blocks with the base-ten blocks.
How many does he have?
Diego took away 2 tens.
Draw a representation to show what happened to Diego’s blocks.
Write an equation to show how many Diego has now.
Then, Diego took away 8 ones.
Draw a diagram to show what happened to Diego’s blocks.
Write an equation to show how many Diego has now. Be prepared to explain your reasoning.
Student Response
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Advancing Student Thinking
Some students may get the ones they need, but also keep the ten. Consider asking:
“Why did you add 10 ones to Diego’s blocks?”
“What is the value of the blocks now?”
“When you decompose a tower of ten, what happens to the tower? How could you show this with the base-ten blocks?”
Activity Synthesis
“What did you need to do with the blocks when Diego took away 8 ones?”
Select previously identified students to share.
“What is different about using base-ten blocks compared to the towers of ten?” (With the towers we could break it apart, but with the base-ten blocks we had to give a ten and take out 10 ones.)
“When you subtract by place, sometimes you need to decompose a ten to subtract ones. When we use connecting cubes, we can show those by breaking apart or decomposing a tower of ten.”
“When we use base-ten blocks, we can't break apart the ten into ones, but we can still show decomposing it by replacing 1 ten with 10 ones.”


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