Got a test tomorrow... Please help me with this stats problem?

The Tyco Video Game Corporation finds that it is losing income because of slugs used in video games. The machines must be adjusted to accept coins only if they fall within certain limits. In order to set those limits, the mean weight of quarters in circulation must be estimated. A sample of quarters will be weighed in order to determine the mean. How many quarters must we randomly select and weigh if we want to be 97% confident that the sample mean is within 0.022 grams of the true population mean for all quarters? Based on results from a sample of quarters, we can estimate the population standard deviation as 0.073 grams.