Nr of customers arriving at a shop follow Poisson. 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)?