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
In this activity, students solve contextual problems that involve subtracting numbers with non-zero digits from numbers with one or more zero digits. Students may choose other ways to find the difference (for example, adding up and using a number line to keep track), but are asked to use the standard algorithm at least once.
In the launch, students subtract their age from the current year. This provides an opportunity for students to notice the relationship between this difference and their current age (MP7).
Required Materials
Materials to Gather
Grid paper
Launch
Groups of 2
Give students access to grid paper.
“Write the year that you were born down and subtract that year from the current year.”
“Share with a neighbor what you found out.” (Students will notice that the number that results is their current age or the age they will be on their upcoming birthday.)
“Jada used this method to find the age of some of her relatives.”
Activity
10 minutes: partner work time
Student Facing
Jada recorded the birth year of some of her maternal grandparents for a family history project.
|   family member   | birth year |
|-------------------|------------|
|    grandmother    |    1952    |
|    grandfather    |    1948    |
| great-grandmother |    1930    |
| great-grandfather |    1926    |
As of this year, what is the age of each family member? Show your reasoning. Use the standard algorithm at least once.
Student Response
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Activity Synthesis
Ask students to share responses.
“Which years did you find easiest to subtract? Which were more difficult? Why?” (Subtracting a year without decomposing any units was easier than subtracting a year that required decomposing.)
Prompt students to check and compare ages found using the standard algorithm and those found using other ways of reasoning.


aligns: no