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Circle Farms wants to develop a replacement policy for its $2-$year$-$old tractor over the next $5$ years. A tractor must be kept in service for at least $3$ years, but must be disposed of after $5$ years. The current purchase price of a tractor is $$40,000$ and increases by $10\%$ a year. The salvage value of a $1-$year$-$old tractor is $$30,000$ and decreases by $10\%$ a year. The current annual operating cost of the tractor is $$1300$ but is expected to increase by $10\%$ a year. Determine the optimal replacement policy of the tractor over the next $5$ years.