Queueing Theory - Probability that all jobs have been served?

Suppose I have M/M/1 system with $\lambda = 4$ per hour and $\mu = 5$ per hour. How can I find out if all jobs have been served after, say, 8 hours? At first I thought about doing $P(n > 40)$ since $\mu = 5$ means 5 arrive, on average, per hour but I'm pretty sure that's not the right way to do it. I don't know how it actually goes, however.

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 $\mu=5$ means 5 are served every hour, not arrived – Alex Dec 31 '12 at 23:50