Assume that McDonalds can serve a customer's order in X minutes after the customer enters the fast food chain. The PDF of the random variable X is shbwn as: Moreover, assume that the customer can finish the food Y minutes after it is served, independent of the serving time. The PDF of the random variable Y is shown as: Let Z = X + Y be the total amount of time that the customer stays inside the fast food chain. i. What is the probability that the customer stays within McDonaids for at most 18 minutes, i.e. PlZ s 13]? ii. What is the expected value (in minutes) that the customer stays within the fast food chain