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The duration of long-distance telephone calls is normally distributed with a mean of 1 8 µ= minutes and a standard deviation of 1 2 ? = minutes

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The duration of long-distance telephone calls is normally distributed with a mean of 1 8 µ= minutes and a standard deviation of 1 2 ? = minutes. If a random sample of 64 telephone calls is used to reflect on the population of all long-distance calls, what is the probability that the sample mean call duration:


(3 marks) will be more than 14 minutes?
(6 marks) will be either less than 15 minutes or more than 20.5 minutes?