standard_description: Derive the formula for the sum of a finite geometric series (when the common ratio is not 1), and use the formula to solve problems. For example, calculate mortgage payments.
problem_activity: Narrative
The purpose of this activity is for students to discuss different ways to solve a three-addend story problem in which none of the addends can be combined to make 10 without being decomposed. Students solve the problem in a way that makes sense to them. Some students may build each number separately on the 10-frames and see both are filled. Some students may manipulate the numbers and think
\(8 + 7 + 5 = 8 + 2 + 5 + 5\)
. Different methods for solving are highlighted in the synthesis. It is important that the teacher represents student thinking, so that these methods are visible to all.
When students make sense of the story problems and connect the quantities in the story to an equation, they reason abstractly and quantitatively (MP2).
MLR6 Three Reads.
Keep books or devices closed. To launch this activity, display only the problem stem, without revealing the question. “We are going to read this story problem 3 times.” After the 1st Read: “Tell your partner what happened in the story.” After the 2nd Read: “What are all the things we can count in this story?” Reveal the question. After the 3rd Read: “What are different ways we can solve this problem?”
Advances: Reading, Representing
Required Materials
Materials to Gather
Connecting cubes or two-color counters
Double 10-frames
Launch
Groups of 2
Give students access to double 10-frames and connecting cubes or two-color counters.
Activity
Read the task statement.
4 minutes: independent work time
2 minutes: partner discussion
Monitor for students who:
place 8, 7, and 5 counters on the 10-frames and see that they are both full
count on
represent the problem as
\(8 + 2 + 5 + 5\)
use a known sum like
\(8 + 8\)
and then add 4 more.
Student Facing
Jada went to the zoo with her family.
They went to the reptile exhibit and saw 8 snakes, 7 iguanas, and 5 frogs.
[IMG]
How many reptiles did Jada's family see?
Show your thinking using drawings, numbers, or words.
Student Response
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Activity Synthesis
Invite previously identified students to share.
“What did they do to find the sum?”
Record each method with an equation.


aligns: no