In a single$-$channel, single$-$phase queuing $($waiting line$)$ system, it is reasonable to assume Poisson arrivals with average arrival rate of $2$ customers per hour. It is also reasonable to assume Negative Exponential service time with average service rate of $4$ customers per hour. Based on these information, the average number of customers waiting in the queue is

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$0\mathrm{.}4$

$0\mathrm{.}5$

$0\mathrm{.}6$

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