5. The Malawi Revenue Authority at Songwe Boarder is equipped with four identical central computers which process the jobs. There are 25 revenue officers connected from their own PC at any given time. Each officer is capable of processing one job via his/her PC every 15 min, on average, but the real time between sending jobs is exponential. The jobs that arrive automatically reach the first available central computer. The execution time per job is exponential and averages 2 min. Calculate the following: a) The percentage of time that the entire calculating centre is not operating. (7 marks) b) The average number of jobs waiting to be executed (6 marks) c) The average time before one job is returned to the officer