Company x has been contracting its overhauling work to company y for $40,000 per machine per year. company x estimates that by building a $500,000 maintenance facility with a life of 15 years and a salvage value of $100,000 at the end of its life, it could handle its own overhauling at a cost of only $30,000 per machine per year. what is the minimum annual number of machines that company x must operate to make it economically feasible to build its own facility? (assume an interest rate of 10%.)