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Monthly credit card payments are calculated here in this percentages and simple interest lesson





Title – Relating Interest to Percentages
By – Mark
Primary Subject – Math
Grade Level – 9
Lesson Time – 1 hour


    By identifying what percentages are, we can use this information to relate it to real life situations. A good way to show that percentages are of use in everyday life would be simple interest. Simple interest is used by most people everyday whether they know it or not. Using simple interest, the students will learn how to calculate monthly payment on a credit card, because most if not all will eventually purchase an item on credit.


    Get the values of certain items the student might want to buy but don’t have money for. Get different interest rates from a couple of credit card companies. Create an excel table of the items, multiple interest rates for each item, and different time periods they might have to pay for the items. Get different interest rates from a couple of credit card companies.


  • created table,
  • calculator,
  • pen/pencil


  • Start by informing the students that percents relate to interest.
  • Tell them how they are going to calculate different interests for different items they might purchase on a credit card.
  • Show how to compute interest using the simple interest formula I=PRT using the principal amount of $1200 at 8% interest for 2 years (=$192).
  • Use the same principal amount and interest rate changing the time to 4 years ($384). Discuss with students the amount of interest they will save if they purchase the items for two years instead of four.
  • Using the examples given to them, calculate what the monthly interest would be for each one. Explain that even though the 2-year is more each month, they are paying more in the end for the 4-year.
  • Have examples at board for them to calculate, select students to do them on the board.
  • Have students get into groups and hand out the created table. Have them calculate each of the items that are empty in the table using the equation for interest.


  • How are percentages and interests related?
  • Where is interest found?
    • Where have they seen it or heard about it?
  • How is interest affected for multiple years?
    • Longer shorter periods, which is better/worse
  • How could this affect their credit if they can not pay this amount per month?
  • What if they go over their limit on a credit card?
    • Penalty fee? More interest?
  • What items do they want that they don’t have the money for?


  1. Have the students complete the handout for homework.
  2. Have them get five different prices on items they want to buy and give them three different interest rates and two different periods for each interest rate for homework.
  3. Have them ask their parents what they think simple interest is and what compound interest is, and bring it to class for discussion for the next lesson.
  4. Have them ask their parents what typical interest rates are for some one who doesn’t have a credit card.

E-Mail Mark !

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