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The life span of LED light bulbs is normally distributed with a mean and a standard deviation equal to 600 and 100 hours, respectively. The life span of fluorescent light bulbs is normally distributed with a mean and a standard deviation equal to 500 and 80 hours, respectively. a) Suppose a sample of four fluorescent light bulbs is selected. Find the probability that the mean life span of the fluorescent light bulbs is less than 643 hours. b) What is the probability that the average life span of 9 randomly selected LED light bulbs is 1 hour more than the average life span of 16 randomly selected fluorescent light bulbs