Statistics

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An industrial engineer has found that the standard household light bulbs produced by a certain manufacturer have a useful life that is normally distributed with a mean of 250 hours and a variance of 2500.
(a) What is the probability that a randomly selected bulb from this production process will have a useful
life between 190 and 270 hours?
(b) Ninety percent of the bulbs have a useful life in excess of how many hours?
(c) What is the probability that the average life of 10 randomly, independently selected bulbs is less than
240 hours?

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