Ryan and Miriam are roommates who take the bus 396 together almost every weekday to go to Down-Under university. Let X be their waiting time at the bus stop (in minutes) before the bus 396 arrives. The random variable X follows a Bus396 distribution with mean 8.9 and standard deviation 2.5. whose density function roughly looks like: Let X1 be their waiting time the first time they take the bus 396. let X2 be their waiting time the second time they take the bus 396, etc. The waiting times at the bus stop are independent. During a term at Down-Under university, Ryan and Miriam take that bus together 39 times. Ryan and Miriam's average waiting time at the bus stop during the term is X1 +X2+X3+ "HI-X89 I": 39