OK, I'm old and this is not so simple for me, and this is not for a test. It's for my own use and knowledge. And I'll greatly appreciate any help.

I'm not sure how to set this up properly in a formula, so I'll do this in a story problem. Here goes:

Joe has a loan of $4,000. His interest rate is 15% (1.25% monthly) and he will pay $100 each month until the loan is paid off. How many months (x) will it take for Joe to pay off this loan?

How would this look set up in the proper formula to find x, and how many months?

Thanks again for any help.