4. At a certain location the annual precipitation is approximately normally distributed with a mean of 45 in. and a standard deviation of 15 in. Annual runoff can be approximated by R = -7.5 + 0.5P where R is annual runoff and P is annual precipitation. What is the mean and variance of annual runoff? What is the probability that the annual runoff will exceed 20 inches? [ use table for normal distribution]