Eventmath/Lesson plans/Using inflation to predict Spotify subscription cost over time

Activities
This lesson plan is based on a statement made in a Tweet by the musician and activist Damon Krukowski of the bands Galaxie 500 and Damon & Naomi.

Initial discussion
I suggest starting the lesson by encouraging some discussion in small groups. Here are some motivating questions you might use:
 * 1) What are "streaming music subscription prices"? What streaming service do you use, and what is the subscription price? (Consider making a list of class responses, perhaps using some polling software to gather results.)
 * 2) How has that subscription price changed over time? Consider looking at your account details or searching the internet for some historical information.
 * 3) Knowing the date when this Tweet was posted (October 13, 2022), why does he prominently mention the idea that "inflation has struck everything..."?
 * 4) What is inflation anyway? Write a one or two sentence definition of what that concept means to you. (Consider asking students to compare their ideas in small groups before sharing with the larger class.)

Mini lesson on inflation and exponential growth
This part will be a lesson on how inflation causes the amount of money required to purchase a specific good to grow exponentially over time. (For consideration: whether to also discuss purchasing power and exponential decay.)

Main task: modeling Spotify's subscription cost over time
Point out that Spotify cost $9.99/month back in 2009 when the streaming service first launched. Then, provide students with an average rate of inflation since 2009 and have them model how that $9.99/month cost would grow over time. Finally, ask them to look up the current monthly subscription rate and compare.

As it turns out, a reply to the original Tweet shows the results of some of these inflation calculations. Ask students to look at that reply Tweet and compare to their findings. Specifically, that follow-up reply shares a screenshot of some text, which is included here for your convenience:

"Value of $9.99 from 2011 to 2022

$9.99 in 2011 is equivalent in purchasing power to about $13.18 today, an increase of $3.19 over 11 years. The dollar had an average inflation rate of 2.55% per year between 2011 and today, producing a cumulative price increase of 31.95%.

This means that today's prices are 1.32 times higher than the average prices since 2011, according to the Bureau of Labor Statistics consumer price index. A dollar today only buys 75.797% of what it could buy back then."

(It appears that this text is auto-generated from some kind of "inflation calculator." Indeed, I believe it comes from this "CPI Inflation Calculator" website. You might consider asking students to find this resource or something just like it!)

Assignments
The main task above used an instructor-provided average rate of inflation for the last N years. As an exploratory assignment, you might ask students to look up how the inflation rate has changed over those years. Could they modify their model to more accurately show the change of that $9.99 value over time?

The main task here was also based on Spotify, specifically, because that's what the original Tweet highlights. You might ask students to look up the subscription rates of other streaming services to see how they have changed over time.

Resources
(Include these sections at your discretion, keeping in mind that you and others can always edit and add more to these sections later.)

Background
''You're welcome to share links to openly-accessible content (e.g. from Khan Academy, YouTube, Wikiversity, etc.) about domain knowledge or math skills students should have. Likewise, you may wish to include contextual information for the instructor.''

Explorations
''You're welcome to share references for additional learning and exploration, such as links to other articles, videos, spreadsheets, or computer code. When an open-access substitute is unavailable, links to paywalled sites are acceptable in this section.''

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