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2. At a small but growing airport, the local airline company is purchasing a new tractor for a tractor-trailer train to bring luggage to and from the airplanes. A new mechanized lug- gage system will be installed in three years, so the tractor will not be needed after that. However, because it will receive heavy use, so that the running and maintenance costs will increase rap- idly as it ages, it may still be more economical to replace the tractor after one or two years. The next table gives the total net discounted cost associated with purchasing a tractor (purchase price minus trade-in allowance, plus running and maintenance costs) at the end of year i and trading it in at the end of yearj (where year O is now). j 1 2 3 i 0 1 2 $8,000 $18,000 10,000 $31,000 21,000 12,000 Management wishes to determine at what times (if any) the tractor should be replaced to minimize the total cost for the tractor(s) over three years. 2.1. Formulate a network model for this problem as a shortest path problem. I 2.2. Formulate and solve a spreadsheet model for this