standard_description: Given a two-digit number, mentally find 10 more or 10 less than the number, without having to count; explain the reasoning used.
problem_activity: Narrative
The purpose of this warm-up is for students to interpret and connect base-ten diagrams and equations and describe how they work. This will be useful when students choose their own representations and methods for subtraction during center activities. Although students may notice and wonder many things about these representations, connections to how the representations show subtracting tens from tens and ones from ones are the important discussion points. As they compare the representations, students develop their skill with place value and operation language  (MP6).
Launch
Groups of 2
Display the image.
“What do you notice? What do you wonder?”
1 minute: quiet think time
Activity
“Discuss your thinking with your partner.”
1 minute: partner discussion
Share and record responses.
Student Facing
What do you notice? What do you wonder?
[IMG]
\(66 - 28 \)
\(66 = 50 + 16 \)
\(16 - 8 = 8 \)
\(50 - 20 = 30\)
\(30 + 8 = 38\)
\(66 - 28 = 38\)
Student Response
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Activity Synthesis
“Both of these representations show ways to find the value of
\(66 - 28\)
. How are they the same? How are they different?” (Both show taking tens from tens and ones from ones. Both show decomposing a ten. One uses base-ten diagrams and one uses only numbers and equations.)


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