standard_description: Solve real world and mathematical problems involving perimeters of polygons, including finding the perimeter given the side lengths, finding an unknown side length, and exhibiting rectangles with the same perimeter and different areas or with the same area and different perimeters.
problem_activity: Narrative
The purpose of this Number Talk is to elicit strategies and understandings students have for adding within 100. These understandings help students develop fluency and will be helpful later in this lesson when students add 2 two-digit numbers, with composing a ten, using methods based on place value and the properties of operations.
In this activity, students have an opportunity to look for and make use of structure (MP7) because they decompose addends to make a new ten.
Launch
Display one expression.
“Give me a signal when you have an answer and can explain how you got it.”
1 minute: quiet think time
Activity
Record answers and strategy.
Keep expressions and work displayed.
Repeat with each expression.
Student Facing
Find the value of each expression mentally.
\(7 + 3\)
\(37 + 3\)
\(7 + 8\)
\(57 + 8\)
Student Response
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Activity Synthesis
“How could
\(7 + 3\)
help you solve
\(37 + 3\)
?” (I know 37 is 3 more tens than 7. Since I know
\(3 + 7\)
is 10, I need to add
\(30 +10\)
.)
“How could
\(7 + 3\)
help you solve
\(7 + 8\)
?” (
\(7 + 3 = 10\)
. So far I have added 3 of the 8 and have 5 left.
\(10 + 5 = 15\)
.)
“How could
\(7 + 8\)
help you solve
\(57 + 8\)
?” (Since I know
\(7 + 8 = 15\)
, I can add
\(50 + 15 = 65\)
)


aligns: no