08 Feb

I am finishing up a unit on ratio, rate, and percent.  I am finally (finally!!) starting to get into the groove of using my class netbooks.  The first time we used Google Docs, it was just a disaster.  The whole class period was spent finding files and troubleshooting login problems.  I stayed away from Google Docs for quite a long time after that.  I created this lesson with an alternate plan for students who couldn’t get Google Docs to work, and I’m glad I did.

Standards for this lesson:
7th Grade Common Core: Use proportional relationships to solve multistep ratio and percent problems (7.RP.3)
Personal Financial Literacy: Solve problems involving percent of a number, discounts, taxes, simple interest, percent increase, and percent decrease (PFL)

Although the standard only mentions simple interest, I think it’s worthwhile to have the discussion about simple vs. compound interest.  The goals of the lesson were:

Describe simple vs. compound interest
Calculate simple interest if given the principal amount and the rate
Discuss how interest is part of saving for goals

I gave the students some notes on vocabulary terms yesterday. We reviewed them briefly today.

Interest_Notes

I told them that today, I wanted them to calculate how much money someone would have if they used simple interest versus compound interest for their savings account.  I wanted them to be able to compare simple versus compound interest, and which one would save you more money.  I pointed students to my Blackboard website and said we’d be working on spreadsheets to do our calculations.  I asked them to discuss the pros and cons of Excel versus Google Docs.  The students were very opinionated about it!
– Google Docs is convenient because you can get your files from home or school
– You can use Excel even if the internet is down
– Google Docs auto-saves whereas you need to remember to save Excel
– You can share Google Docs with someone to get help

Most students favor Google Docs.  However, not all students were able to view the file I shared, and I think it has to do with the settings for viewing their Google Drive… but I couldn’t troubleshoot it then and there.  I told students who couldn’t see my file to please open Excel. We spent some time just getting the files to open.

I guided them through making the calculations for Simple Interest, where you have \$5000 in the bank and get paid 4% interest every year. Every year, someone would earn 4% of \$5000 and still have \$5000 in the bank.  The money was paid out directly and not put back into the principal.  The students decided that after the sixth full year (seven interest payments), the person would have earned \$1400 in interest and still have \$5000 in the bank, for a total of \$6400.

I started them on the calculations for Compound Interest.  Students calculated 4% of the balance, added the interest to the beginning balance, and then this became the balance for the next year.  Next year, the bank account had more money and would earn 4% on that larger amount, and then that interest got added in, and so on.  I let the students finish with help from their friends.  A few students got stuck and stayed stuck, however most were quite persistent and sought help from friends to get the math done.  Some were really savvy about sharing their documents to troubleshoot together.  I felt good, as this was a great use of technology in the classroom and I think we are getting good bang-for-the-buck from the netbooks.

Some students were able to start on the second interest scenario, but many did not get there.  We discussed the differences between simple and compound interest, and we noted that compound interest earned more money over the years.  This was very easy to tell from the spreadsheet. Doing the calculation for each year was powerful.

I regret that I did have some students that stayed stuck on the task.  I will be targeting them over the next few lessons.

I enjoyed the task and I think the kids learned a lot from it.  I am attaching the blank spreadsheet, the key spreadsheet, and just for fun, my ticket-out-the-door responses (without names).  What do you think?  Did they get the idea behind the lesson?  Clearly I have some more spiraling to do on this topic, but for the first time they’ve calculated an interest scenario, I was pleased with the progress.

InterestLessonBlank  : Blank spreadsheet in Excel format