Are you allowed to use a spreadsheet to work out this answer? It's the easiest way.
If not, you'll have to let the monthly payment be x, and then say that, at the start of each new year, the amount owing
= amount owed at the start of the last year + interest payable during that year - 12x
Then work out how much interest he'll have to pay in the coming year to cover this new amount owing. Then repeat for the following year, and so on, up to the start of year 16. Then say that at the start of year 16 he owes nothing.