A man gets a credit card and buys something that charges exactly 800 dollars to the card. The APR on the card is 18 % compounded monthly, and the minimum payment is 15 dollars a month. What is the expression for A(n), the balance on the card after n months? (This should be a geometric series). I have tried to come up with different formulas, and none provide answers that make sense. I have tried to fit the information into formulas combining the sum of a geometric series and compound interest, with no luck.