The purpose of this task is to introduce the idea of interest, and show through example how the balance is different if someone earns interest on their interest, versus someone who withdraws the interest every year. The activity asks a question without much scaffolding, which offers an opportunity for students to make sense of the problem and persevere in solving (MP1).

In addition to “interest,” students may need help interpreting other terms, like “deposit” and “withdrawal.” Use the Launch to gauge how much time to spend on helping students understand the terms associated with bank accounts.

Monitor for different ways students organize their work.

The goal of this discussion is to compare the result of earning interest on interest and only earning interest on the original amount, as well as showing ways of organizing approaches to this situation. Select students to share their work who organized it clearly. For example, the information and calculations could be organized in tables, like this:

The important point to draw out is that Jada earned interest on her interest, and Kiran did not. Here are some questions for discussion:

• “From their original deposit of $1,000, how much did Kiran have at the end of year 2?” ($1,150.)
• “How much did Jada have at the end of year 2?” ($1,155.83.)
• “Why did Jada end up with slightly more money in year 2? (She earned 7.5% on $1,075 since she left her interest in the bank. But Kiran earned 7.5% on only $1,000 since he withdrew the interest from the bank. While 7.5% of $75 is not much, Jada did have more money in year 2.)

In this practice activity, students have an opportunity to solve a problem in which a percentage change is applied repeatedly. After students have had a chance to make a visual display, conduct a gallery walk or invite one group for each problem to present their solution. When students explain why they agree or disagree with a character’s statement, they are justifying their reasoning and critiquing the reasoning of others (MP3).

The goal is for students to discuss the situations and calculate repeated percentage change. Much of the discussion will occur within the groups. If time permits, consider conducting a gallery walk. Post each group’s visual display around the room, and give every student a few sticky notes to add questions or comments to the displays.

Alternatively, invite one group for each problem to present their work to the class. To keep other students involved, ask:

• “Can you restate or summarize this group’s approach?”
• “Think about this group’s solution. What do you agree or disagree with?”
• “What are some choices the group made that made their display easy to understand? What are some things they could improve?”