Answer:
86,428 - 70,000 = $16,428
This difference of $16,428 refers to the 9% interest that was paid over the 4 years. However, the 9% is only charged on the amount that is owed whch reduces every year by a principal repayment which also comes out of the $21,607.
Year 1
Payment = $21,607
Interest = 9% * 70,000 = $6,300.
Principal repayment = 21,607 - 6,300= $15,307
Amount left to be paid = 70,000 - 15,307 = $54,693
Year 2
Payment = $21,607
Interest = 9% * 54,693 = $4,922.37
Amount left to be paid = 54,693 - (21,607 - 4,922.37) = $38,008.37
Year 3
Payment = $21,607
Interest = 9% * 38,008.37 = $3,420.75
Amount left to be paid = 38,008.37 - (21,607 - 3,420.75) = $19,822.12
Year 4
Payment = $21,607
Interest = 9% * 19,822.12 = $1,783.99
Amount left to be paid = 19,822.12 - (21,607 - 1,783.99) = $0
Interest Year 1 - 4 = 6,300 + 4,922.37 + 3,420.75 + 1,783.99
= $16,427.11 (difference due to rounding errors)