Consider a demand or load versus capacity or strength relation, z = r - s, where r denotes the capacity and s the demand. assume that capacity is described by a normal distribution with r  = 200, r = 40, and the demand is also described by a normal distribution with s = 100, s  = 30. a) what is the distribution of z? b) estimate the mean and standard deviation of z; c) calculate the reliability index  and probability of failure pf.