I expect there is some potted formula for theis, but we can also work it out

from first principles.

Let the principal be and be one more than the monthly interest rate, and the monthly repayment be .

Then after 1 month our outstanding debt is:

after 2 months:

and after months:

Now 30 years is 360 months and after this time our debt will be zero so:

or:

RonL