Verrrry Interesting!

Teacher Notes:

• As an extension, have students choose different balances and different interest rates and find how long it takes to pay off their chosen balances. Some credit card companies even offer lower interest for the first few months. Students could include this idea in the problem they choose to solve.
• Students could also choose to make more charges each month and add that to the situation they chose above.

Procedure for the Activity

Answers:

Step 1  Answers will vary.

Steps 2–3

Step 4  B = 1 × (500 – 15) + 0.015(500 – 15); B = 1 × 485 + .0.015(485) = 485 + 7.275 = 492.275.

Step 5  B = 1 × (500 – 15) + 0.015(500 – 15) = (1 + 0.015)(500 – 15) = 1.015(485); 1.015(485) = 492.275 » 492.28

Step 6  B = 1 × (a – 15) + 0.015(a – 15) = (1 + 0.015)(a – 15) or 1.015(a – 15)

Step 7  Sample table: Students may choose to use different columns than in this table.

Step 8  See students’ work.

Step 9  There were 46 payments of $15 and a final payment of $8.58. That is 46(15) + 8.58 = 570.69. Therefore, $698.58 – $500.00 = $198.58 in interest was paid.

Wrapping Up the Activity

See students’ work.