the school of business has 3 fax machines. the toner in each machine needs to be changed after about 5 hours of use. there is one unit secretary who is responsible for the fax machine maintenance. it takes him 15 minutes to replace the toner cartridge. what is the average time spent in the system? a. 0.2749 hrs b. 0.0142 hrs c. 0.1563 hrs d. 0.0249 hrs e. none of the above