please need help solving this probelm. dont know where to start, and have to show work.

A railroad branch line to a landfill site is to be con- structed. It is expected that the railroad line will be used for 15 years, after which the landfill site will be closed and the land turned back to agricultural use. The railroad track and ties will be removed at that time. In building the railroad line, either treated or untreated wood ties may be used. Treated ties have an installed cost of $6 and a 10-year life; untreated ties are $4.50 with a 6-year life. If at the end of 15 years the ties then in place have a remaining useful life of 4 years or more, they will be used by the rail- road 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 ser- vice life to be used elsewhere, can be sold for 50 each. (a) Determine the most economical plan for the ini- tial railroad ties and their replacement for the 15-year period. Make a present worth analysis assuming 8% interest. (b) Plastics in landfills do not degrade quickly! and the land may not be useful for agriculture. Research and write a summary of what is being done with landfills now to make them more earth friendly and reduce negative impacts