Assume that you are considering replacing your old Nissan with a new Hyundai. The price of your favorite Hyundai model is $18,000, and it is expected to run for 15 years. However, the annual maintenance cost of the old Nissan increases as time goes by. It is $1,200 in the first year, $1,500 in the second year, and $1,800 in the third year. When should you replace it with the new Hyundai in this case?