$4.$ A bus company believes that it will need the following number of bus drivers during each of the next five years: year $160$ drivers; year $270$ drivers; year $350$ drivers; year $465$ drivers; year $575$ drivers. At the beginning of each year, the bus company must decide how many drivers should be hired or fired. It costs $$4,000$ to hire a driver and $$2,000$ to fire a driver. A driver$$s salary is $$10,000$ per year. At the beginning of year $1,$ the company has $50$ drivers. A driver hired at the beginning of a year may be used to meet the current year$$s requirements and is paid full salary for the current year. Formulate an LP to minimize the bus company$$s salary, hiring, and firing costs over the next five years.