I'm trying to help my son with his homework but this has me stumped; the answer is in the back of the book but I've no idea how to arrive at it.

For this loan: ......

principal + interest = $2835

was repaid over a Period of 15 months

and pays Interest at a rate of 4% per year

What was the principal of the loan?

Any idea what steps you would use to solve this?

Thanks in advance