7.39 A certain type of elevator has a maximum weight capacity X1, which is normally distributed with a mean and standard deviation of 5,000 and 300 pounds, respectively. For a certain building equipped with this type of elevator, the elevator loading, X2, is a normally distributed random variable with a mean and standard deviation of 4,000 and 400 pounds, respectively. For any given time that the elevator is in use, find the probability that it will be overloaded, assuming X1 and X2 are independent.