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GHR is a cell-phone production firm. It produces 25 types of cell-phones, which are categorized into two classes: A and B. In the following table, forecasts for demand of different phone classes and the number of working days for the next six months are given. There are currently 36 full time workers and each worker produce 4 phones per day (same for the both classes). At the beginning of each month, new full-time workers are hired at a cost of $400 and existing workers can be fired at a cost of $580. Wages consist the majority of the total cost and workers are paid $100 per day including insurance, meal and transportation. In other words, wages may change from one month to another month. Demand must be satisfied on time. Assume that there is no back orders and lost sales. The inventory holding cost is $4 per item per month. Inventory level of Class A products is 485 at the beginning of January. TABLE 1. Demand data for Product Classes and number of working days for different months a) Generate the formula of optimization model to minimize total cost, and show production amounts for each month; numbers of hired and fired workers for each month and the value for total cost. Specify decision variables, objective function, and constraints based on the given data/information. (12 points) b) Solve this optimization model using Excel Solver and analyze your results. ( 8 points)