6. (15 pts) The life of a semiconductor laser at a constant power is normally distributed with a mean of 6000 hours and a standard deviation of 600 hours. a. (2 pts) What is the probability that a laser fails before 4,800 hours? b. (2 pts) What is the life in hours that 90% of the lasers exceed? c. (3 pts) What should the mean life equal in order for 99% of the lasers to exceed 10,000 hours before failure? d. (3 pts) A product contains three lasers, and the product fails if any of the lasers fails. Assume the lasers fail independently. What should the mean life equal in order for 99% of the products to exceed 10,000 hours before failure? e. (5 pts) Rework parts (a) and (b) only. Assume that the lifetime is an exponential random variable with the same mean