options: Add and subtract within 1000, 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. Understand that in adding or subtracting three-digit numbers, one adds or subtracts hundreds and hundreds, tens and tens, ones and ones; and sometimes it is necessary to compose or decompose tens or hundreds.

Explain why addition and subtraction strategies work, using place value and the properties of operations. (Explanations may be supported by drawings or objects.)

Fluently add and subtract within 100 using strategies based on place value, properties of operations, and/or the relationship between addition and subtraction.

Add up to four two-digit numbers using strategies based on place value and properties of operations.

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 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.”


correct_option_index: 2