The number of telephone calls that arrive at a phone exchange is often modeled as a Poisson random variable. Assume that on the average there are 20 calls per hour. Round your answers to four decimal places (e.g. 98.7654). (a) What is the probability that there are exactly 18 calls in 1 hour? (b) What is the probability that there are 3 or fewer calls in 30 minutes? (c) What is the probability that there are exactly 30 calls in 2 hours? (d) What is the probability that there are exactly 10 calls in 30 minutes?

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