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Statistics - Confidence Interval

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A statistics professor wants to compare today's students with those from 25 years ago. All of his current students' marks are stored on a computer so that he can easily determine the population mean. However, the marks from 25 years ago reside only in his paper files. He does not want to retrieve the marks and will be satisfied with a 95% confidence interval estimate of the mean mark of 25 years ago. If he assumes that the population standard deviation is 12, how large a sample should he take to estimate the mean to within 2 marks?

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