The customer services department of a large retailer receives at least approximately 1200 emails from customers per day, all of which have to be replied to that day. The department employs two grades of staff: junior and senior. Both grades work for 8 hours per day, but junior staff can only process emails at a rate of 8 per hour compared to the senior rate of 10 per hour. Each member of staff requires a computer terminal and the department has a maximum of 25 terminals available. There is also a limited amount of desk space available (35m2). Each senior member of staff is expected to supervise junior staff, and hence senior staff are allocated more space (2m2) than junior staff (1m2). In order to ensure that proper supervision is always available, there must be at least 2 members of senior staff to every five junior members of staff. Taking into account all direct and indirect costs, the cost of employing a senior member of staff is $15 per hour, compared to $10 per hour for a junior member of staff. Develop an LP model for this problem and solve it using Excel solver. Then, upload your file, which must contain the following: 

Required: 

1) Definition of decision variables, 

2) LP model (Symbolization stage only), 

3) The optimal solution in Excel and 

4) Practical interpretation of the solution