Practicing some C need help

Hello, so I decided to start doing some C programming again to help refresh my memory. I decided to work on an amortization table for a loan. I found an example online of writing a program for calculating the interest rate for someone who buys a TV for 1000$ at an 18% interest rate with no down payment and a monthly payment of like 50$. I don't want to hard code the numbers.The program starts by asking the user for four numbers: The selling price, down payment, annual interest rate and the payment size.The program then displays a table like this that I have wrote up (with all payments displayed):

Pay #  Old Balance   Payment  Interest  To Balance  New Balance

-----      ----------------  ------------   ----------    --------------  ------------------

   1       1000.00          50.00         15.00          35.00             965.00

   2        965.00           50.00         14.48          35.52             929.48

  23         95.69           50.00         1.44            48.56             47.13

  24         47.13           47.84         0.71            47.13             0.00

-----------------------------------------------------------------------------------------

Total of payments =      1197.84

Total of interest =       197.84

Total to balance  =      1000.00

I have an old program I wrote up a few years back that's actually very similar but it's slightly different, asking the user to enter in the term of the loan and now I don't need them to do that. Though I can't recall that program working correctly. Can I use my old program that I have but change some things around, like the loop and variables, to get it to do what I want now? I'll post the program I have in a pastebin link. Any help would be great.

http://pastebin.com/kam7sQE5