12. In the central railway station 15 computerised reservation counters are available. A customer can book his/her ticket in any train on any day in any one of these computerised reservation counters. The average time spent on each customer by each clerk is five minutes. Average arrivals per hour during three types of activity periods have been calculated and customers have been surveyed to determine how long they are willing to wait during each type of period.

Making suitable assumptions on this queuing process, determine how many counters should be kept open during each type of period. (CA, Nov. 1991)

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Type of period Arrivals per hour Customers' acceptable waiting time Peak 110 Normal 60 Low 30 15 minutes 10 minutes 5 minutes