OK, I first thought it was spam, with such subject...
I appreciate the last sentence. But I moved the topic, as I consider Programming Questions to be about code you already wrote. I move here such generic, open ended questions, PQ are more for questions about specific code.

I don't know much about finance, so I have no idea about this "total interest charged on a credit card before it is paid off". Do you have a formula? If not, perhaps you can do some Internet search? What inputs the function need? I suppose it can be an amount of money, an interest rate, a duration, such things...

Well, you would need to know the initial balance, the interest rate being charged each month and the amount being paid off each month, and you'd need a separate variable to store the total interest. Each iteration, subtract the payment from the balance. If the balance >0, work out the interest charged and add it to the balance and to the total interest charged. Rinse and repeat until the balance <=0, then return the total interest charged.