A reactor of special design un the major item of equipment in a small chemical plant. The initial cost of a Completely installed reactor is $ 60000 to the salvage value of the end of the useful life is estimated to be $ 10000. The total annual expenses for the plant are $100000. Excluding depreciation costs for the reactor. How many years of useful life should de estimated for the reactor if 12% of the total annual expenses for the Plant are due to the cost for reactor depreciation