# How to calculate interest on a car loan?

Formula: r = p * n / m - 1166 + 0.83

p = $5,000 n = 60 (Months) m = $200 (Monthly Payments).

If I put p = $5,000 it's ok. But when I put p = $13,000 it overflows giving me the wrong interest rate.

Is this a question about the (non)sense of a formula (which combines letters with two random numbers of different sizes - which is the meaning of 1166 and 0.83)? Which is the interest rate we ar searching for? Is it an annual rate or a monthly rate? Do we use exponential growth (or some linear approximation)?

If the result is the wrong interest rate (well...) which is the good one? How is this related to sage?

(My questions above are no joke, it is always important to ask a self contained question, yours, because this is half of the solution. The many questions it raises, you saw above, the less understood is the situation. A question is the important part of a communication, it makes 90% of it. The other 90% come from a short answer - when the question hits the point. I had no chance.)

Note that this website is about Sagemath, a computer algebra system, you might be confusing with Sage, an accounting and business management software.