Yes, that looks reasonable, but I was given test criteria to ensure the formula is correct. If the monthly payment is 165.25, the total number of payments is 36, and the interest rate is 9%, the balance after the first payment is 5070.31. When I run these numbers I get a first payment of 1746.16.

Any ideas? Thanks for helping me dig.

I think your problem is you are asking for the annual interest rate which is 9% and the calculation uses the monthly interest rate which is .0075

using your data with this value a run gives

Enter the monthly payment:

165.25

Enter the annual interest rate:

.0075

Enter the number of months required for pay off:

36

After payment number Payoff

1 $5070.308952032767

2 $4943.086269173012

3 $4814.909416191807

4 $4685.771236813245

5 $4555.664521089341

6 $4424.582004997514

7 $4292.516370034991

8 $4159.460242810253

9 $4025.4061946313295

10 $3890.3467410910625

11 $3754.2743416492444

12 $3617.1813992116136

13 $3479.0602597057

14 $3339.9032116534904

15 $3199.70248574089

16 $3058.450254383945

17 $2916.138631291825

18 $2772.7596710265116

19 $2628.3053685592095

20 $2482.7676588234012

21 $2336.1384162645772

22 $2188.40945438656

23 $2039.572525294457

24 $1889.619319234164

25 $1738.5414641284187

26 $1586.3305251093814

27 $1432.978004047701

28 $1278.4753390780572

29 $1122.8139041211405

30 $965.985008402048

31 $807.979895965062

32 $648.7897451847983

33 $488.40566827368485

34 $326.8187107857351

35 $164.01985111662685

36 $0.0