4. An email inbox receives messages at an average rate of ? = 4 per hour, with arrivals independent and the rate stable. Let Z be the number of emails in a specified window. (a) (4 points) Find P(Z = 6) for the next 2 hours. (b) (4 points) Find P(Z ? 1) for the next 15 minutes. (c) (4 points) State the mean and variance of Z for each window above.