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I'm working through some practice exercises for my probability course and I think there's a concept I'm not totally understanding.

The question I'm trying to work through is as follows:

The number of mail a post office receives in one day is defined by a Poisson random variable with parameter $\lambda = 5$. The post office fails to process a piece of mail with probability $0.04$ independently with every ball. Find the probability that the post office processes exactly 5 pieces of mail in a working day.

From what I understand, we can model the number of calls as $\dfrac {(e^5)5^{10}} {10!}$ but how do I use the rest of the information to determine the probability mass function?

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    _"The post office fails to process a piece of mail with probability 0.04 independently with every ball."_ What does that mean, "with every ball"?2012-11-11
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    i think it's: "The post office fails to process a piece of mail with probability 0.04 independently with every mail received."2012-11-12

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