Three machines are employed in an isolated area. They each produce 2,000 units of output per month, the first requiring $20,000 in raw materials, the second $25,000, and the third $28,000. What would you expect to be the monthly charge for the first and second machines if the services of the third machine can be hired at a price of $9,000 per month? Which parts of the charges for the first two machines are economic rent?