interest due at the end of 120 days
= 8500(120/365)(.09) = 251.51
so of his $4000 payment, $3748.49 will go towards reducing the debt.
so balance owing = 8500 - 3748.49
= 4751.51
interest for the remaining 80 days
= 4751.51(.09)(80/365) =93.73
total owing at the end = 4751.51+93.73
= $4845.23