# Credit Card Investigation: What is interest? (Day 2 of 4)

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## Objective

SWBAT write an exponential growth or decay function to model a given real world situation.

#### Big Idea

On day 2 students find percent increase/decrease and simple interest to establish a pattern which extends to writing exponential functions.

## Introduction and credits: Credit Card Investigation

I found this lesson on the blog Function of Time. I love how the activity is designed to step students into modeling with exponential functions. The students work in a real world context that is soon to be applicable to our students. AND, the lesson is a deep exploration of the mathematics. Thank you to Blogger (Kate Nowak) for sharing such a great activity! Love it!

Environment: This credit card investigation is scheduled to last over 3 class periods (later revised to 4 class periods). I will outline how I plan to split this up, but of course it may not tie up as nicely as I would like each day. I am going to have students work on this activity ONLY to maximize class time. No clicker questions for warm-ups and closures. I really want them to focus on their work and make connections. We will summarize and close out the learning together daily.  I want students working in teams to solve these problems and will be asking students to focus on MP3: Construct viable arguments and critique the reasoning of others.  I plan to have whiteboards and markers available to students for scratch work and brainstorming.

## Day 2 Credit Card Investigation

50 minutes

Students should finish part 2 from yesterday's lesson before moving on to part 3.

Part 3: Skipping and making payments

In this section students will explore how making a \$50 payment vs. a \$20 payment every month will affect the credit card balance. Emphasize to students that this isn't really how credit card interest is calculated (once a year), but we will explore more tomorrow how interest is really calculated. ﻿

Part 4: Applying Knowledge

In this section, students will be required to apply their results from parts 1 through 3 of the activity. This is a great check to see if students understood the material. This section could be assigned as homework if students did not finish it in class in addition to homework #5.

## Homework

Assign worksheet #5 from this unit for homework. Students are just being asked to watch a 20 min podcast interview from NPR. So, students will need a computer and internet access to complete this assignment. It may be helpful to include this link on a class website or shared drive. You can also download the podcast from the NPR website so that students can upload it to their music players.