Amount borrowed, A1 = 85,000
Rate, i, for the first 7 years = 13.5% = 0.135
Rate, i, for the remaining 18 years = 15% = 0.15
Part (a): Amount owed after 7 years
Without interest rate, monthly payment, Po= 85,000/(12*25) = 283.33
After 7 years, principal amount remaining = 85,000 - (283.33*7*12) = 61,200.
Therefore, the amount owed after 7 years, A2 = 61,200
Part (b): Monthly repayment for the rest of 18 years
Rate, i = 15% = 0.15
Monthly payment, P(18) = A1*D2
D2 = {0.15/12}/{1-(1+0.15/12)^-12*25} = 0.01281
Then,
P(18) = 85,000*0.01281 = 1088.71
Part (c): Total interest paid
I =[ I(7) + I(18)] - A1
I(7) = P(7)*7*12 + P(18)*18*12
Now, P(7) = A1*D1
D1 = {0.135/12}/{1-(1+0.115/12)^-12*25} = 0.01165
Then,
P(7) = 85,000*0.01165 = 990.25
Therefore,
I = [990.25*7*12 + 1088.71*18*12] -85,000 = 318,341.50 -85,000 = 233,341.50