so you are saying that i should label

Question

p = 30
i = 0.0125/12 = 0.0010416666666666666666666666666667
n = 12(3) = 36

then plug it into what formula ?

Problem:

Stereo shack sells a stereo system for $600 down and monthly payments of $30 for the next 3 yrs. In the interest rate is 1.25% per month on the unpaid balance, find the cost of the stereo system.

Answer:
$1465.42

For Further Reading

math, question - drwls, Friday, June 22, 2007 at 2:44pm
The amount of loan that gets amortized at $30 a month and 1.25% monthly interest is $865.42. Add the down payment to that and you get the original price.

There is no easy formula to plug into. You need an amortization worksheet. I thought I provided a link to one:
(Broken Link Removed)

Leaver the principal blank: it will calculate it. The annual interest rate is 15%; the payment is $30 and the number of payments is 36. Hit "enter" and you will get the financed amnount, which is remainder of the purchase price after down payment