The Fly By Night Used Car Lot uses the following to illustrate their 8.1% finance plan on a car paid over 3 years.

Cost of the car:8.1% finance charge:Total cost:Monthly payment:$10800$2624.4(that is, 8.1%/year$108003 years)$13424.4$13424.436=$372.9.Cost of the car:$108008.1% finance charge:$2624.4(that is, 8.1%/year$108003 years)Total cost:$13424.4Monthly payment:$13424.436=$372.9.

The first payment must be made on the date of purchase. Moreover, the dealer will pay you back 5% of the cost of the car at the end of 4 years if you return it. What is the true interest rate compounded monthly being charged?

The answer to this problem is 13.47%, please show how you would get to this answer.