# Solve: Sample size Needed to Estimate a Population Mean.

A consumer advocacy group is doing a large study on car rental practices. Among other things, the consumer group would like to estimate the mean monthly mileage of cars rented in the U.S. over the past year. The consumer group plans to choose a random sample of monthly U.S. rental car mileages and then estimate using the mean of the sample.

Using the value 750 miles per month as the standard deviation of monthly U.S. rental car mileages from the past year, what is the minimum sample size needed in order for the consumer group to be confident that its estimate is within 135 miles per month of the population mean?

Carry your intermediate computations to at least three decimal places. Write your answer as a whole number (and make sure that it is the minimum whole number that satisfies the requirements).

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#### Solution Summary

This solution shows how to use the formula for a confidence interval to get the formula for the sample size when estimatinga population mean with a confidence interval.