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Sample Size for Average ATM Cash Withdrawal Study

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Last year a study showed that the average ATM cash withdrawal took 65 seconds with a standard deviation of 10 seconds. The study is to be repeated this year. How large a sample would be needed to estimate this year's mean with 95 percent confidence and an error of +-4 seconds?

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This solution provides the simple formula and step-by-step calculations for sample size.

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