William and Ken ride separate buses to the city center in order to meet. Their buses start at the same time. William's travel time has normal distribution with mean 20 minutes and stan dard deviation 3 minutes. Ken's travel time has normal distribution with mean 22 minutes and standard deviation 3 minutes. The travel times are independent. Let X denote how much Ken arrives after William (negative if Ken arrives rst). 1.) What is the probability that one of them has to wait more than 3 minutes