standard_description: Understand subtraction as an unknown-addend problem. For example, subtract 10 – 8 by finding the number that makes 10 when added to 8.
problem_activity: Narrative
The purpose of this activity is for students to use their knowledge of base-ten diagrams and place value to make sense of a written method. They describe how numbers, words, and equations can be used to represent the steps they have used with other representations (MP2). Students will explore other ways to use equations, including the standard algorithm, in future grades. Although some grade 2 students may begin to use recording methods like the standard algorithm, it is important to encourage students use the methods that make the most sense to them.
Required Materials
Materials to Gather
Base-ten blocks
Launch
Groups of 2
Give students access to base-ten blocks.
Display Elena’s work.
“Take a minute to make sense of Elena’s subtraction.”
1–2 minutes: quiet think time
“Discuss Elena’s work with your partner.”
1–2 minutes: partner discussion
Share and record responses. Highlight responses that describe the numbers Elena is subtracting and how Elena shows decomposing units.
Activity
“Elena is finding the value of
\(726-558\)
. Use base-ten blocks or a base-ten diagram to show Elena’s steps. Then finish Elena’s work. If you have time, work together to show a different way Elena could use numbers or equations to show her steps.”
7 minutes: partner work time
Monitor for student examples that record in a way similar to Elena, but use numbers to show the value of each place.
Student Facing
Elena’s thinking:
Step 1:
[IMG]
Step 2:
[IMG]
Step 3:
[IMG]
Use base-ten blocks or a base-ten diagram to show Elena’s steps.
Finish Elena’s work to find the value of
\(726-558\)
.
What is another way you could use numbers or equations to show subtracting by place to find the value of
\(726-558\)
?
Student Response
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Advancing Student Thinking
If students rely on one way to show subtracting by place with numbers or equations, consider asking:
“How did Elena represent the value of each digit in 726 and 558?”
“What is another way you could represent the value of the digits and show subtracting by place?”
Activity Synthesis
Display a student representation of Elena’s steps with a base-ten diagram and 1–2 other representations that use numbers and equations.
“How are these representations the same? How are they different?” (They all show decomposing a hundred and a ten before you subtract. They all show subtracting hundreds from hundreds, tens from tens, and ones from ones. They use different ways to represent each unit. One uses base-ten diagrams, one uses numbers and words, and one just represents the value of each place.)


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