4-83. The life of a semiconductor laser at a constant power is normally distributed with a mean of 7000 hours and a standard deviation of 600 hours. (a) What is the probability that a laser fails before 5000 hours? (b) What is the life in hours that 95% of the lasers exceed? (C) If three lasers are used in a product and they are assumed to fail independently, what is the probability that all three are still operating after 7000 hours