The life X of the wing spar of a certain airplane is normally distributed with a mean of 20,000 hours of flight time and a standard deviation of 1800 hours of flight time. 

(a) Calculate the s-score of a wing spar life of 22,000 hours
(b) Calculate the probability that the life of a wing spar is less than 21,000 hours
(c) How likely is it that the life of a wing spar will be greater than 18,500 hours
(d) Calculate the 90th percentile of wing spar lives.