5- A stearn boiler is needed as part of the design of a new plant. 63 The boiler can be red by natural gas, fuel oil, or coal. A cost analysis shows that natural gas would be the cheapest at $30,000; for fuel oil it would be $55,000; and for coal it would be $180,000. If natural gas is used rather than fuel oil, the annual fuel cost will decrease by $7500. If coal is used rather than fuel oil, the annual fuel cost will be $15,000 per year less. (:1) Assuming 8% interest, a 20year analysis period, and no salvage value, which is the most economical installation