Here we have data from an Internet mortgage calculator. Imagine borrowing $100,000 for fifteen years at 5%.

(We used this calculator, but there are a lot out there. We needed to do a small amount of text processing to get rid of the commas and dollar signs.)

month is the time variable; what month are we in?
interest is how much you pay in interest that month.
principal is how much you pay to the principal that month.
balance is the balance of your loan after your payment.

What is your monthly payment?

What function models principal well?

What do that function's parameters mean?

Can you tell whether the bank is rounding?