It's a math problem relating to sales of differnt computer models.
Shouldn't be that hard to delete and modify it to your needs if you know the equation. I think I have a few more that are geared more to what you are doing.
L = amount of loan
i = monthly interest rate (so assume the APR has been divided by 12 already)
n = number of payments
R = amount of each level payment
Then
R = L * i / (1 - v^n)
where v = 1/(1+i)
total amount of all payments = n * R
total interest paid = n * R - L