Dean Smith is dissatisfied with the time it takes to get a new faculty ID made and believes more servers will speed up s

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Dean Smith is dissatisfied with the time it takes to get a new
faculty ID made and believes more servers will speed up service and
reduce costs thanks to shorter lines. Prof. Karen is tasked
to study the situation. Prof. Karen observes an average of 20
customers/hr arriving and each technician (service window) serves 5
customer/hr, on average. Assume an M/M/s queue system.
Prof. Karen calculates the operational cost of each server as
$20/hr, and assumes a cost of waiting in the SYSTEM as $25/hr per
customer.
What is the optimal number of service windows to minimize total
cost?