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Sample size and margin of error in a confidence interval for a mean

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Jeremiah wants to estimate the mean age of students at his large university. He'll sample n students and make a 95% confidence interval for the mean age. He is willing to use σ=3 years as an estimate of the standard deviation, and he wants the margin of error to be no more than 1 year.
Which of these is the smallest approximate sample size required to obtain the desired margin of error?
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