Sara has graduated from HS and her parents have given her a car fund of $21,000 to help purchase and maintain a 3 year old car in college. Because of maintenance costs, her parents tell her that she may trade in her car for another 3 year old car (at a cost of $12000), one or more times during the next 3 summers if she determines that it will minimize her overall costs. They also tell her that in 4 years they will buy her a new car as a college graduation present so she should definitely plan on trading in the car then. Yearly operational and maintenance cost are provided below. Formulate the problem as a minimum flow network problem and then solve it using excel.

	Operating and Maintenance Costs for Ownership Year				Trade in Values at End of Ownership Year
Car Purchase Price	1	2	3	4	1	2	3	4
$12,000	$2000	$3000	$4500	$6500	$8500	$6500	$4500	$3000