5-61 A railroad branch line to a missile site is to be con. structed. It is expected that the railroad line will be used for 15 years, after which the missile site will be removed and the land tumed back to agricultural use The railroad track and ties will be removed at tha time. In building the railroad line, either treated on untreated wood ties may be used. Treated ties have a installed cost of $6 and a 10-year life; untreated tie: are $4.50 with a 6-year life. If at the end of 15 year the ties then in place have a remaining useful life of 4 years or more, they will be used by the railroad elsewhere and have an estimated salvage value of $3 each. Any ties that are removed at the end of their service life, or too close to the end of their service life to be used elsewhere, can be sold for $0.50 each. Determine the most economical plan for the initial railroad ties and their replacement for the 15- year period. Make a present worth analysis assuming 8% interest