I need a formula

I’m sure it’s out there, and I’m sure my Husband could come up with one. But I’ll have the debt paid off before he gets down researching it, figuring it out, and explaining it!

How much will you save if you throw a one-time extra $50 on a debt on a debt with 10% interest that you will pay off in a year. I know you have three variables — $50, 10% and 12 payments. I just don’t know what to do with those three numbers.

Sincerely, Helen.