- #1

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In 15, an average of 4 customers arrive.

a)

A customer has just arrived. Then a minute passed and no one arrived. What is the probability of it takoing at least 5 more min. until another customer arrives?

b)

Consider 40 non-overlapping periods of 15 min.

What is the probability that

at least 7 and at most 15 of those intervals have at most 2 customers arriving?

In book, answer to

a) is 0.2636

b) is F(2.22) - F(-1.12) = 0.855

In a) although I don't see why, I understande that it's something about memorylessness or something. But how do you get to the answer in question b)?