Customers arrive at a movie theater at the advertised movie time only to find that they have to sit through several previews and prepreview ads before the movie starts. Many complain that the time devoted to previews is too long. A preliminary sample conducted byThe Wall Street Journalshowed that the standard deviation of the amount of time devoted to previews was 4 minutes. Use that as a planning value for the standard deviation in answering the following questions. Round your answer to next whole number.

a.If we want to estimate the population mean time for previews at movie theaters with a margin of error of75seconds, what sample size should be used? Assume95%confidence.

b.If we want to estimate the population mean time for previews at movie theaters with a margin of error of1minute, what sample size should be used? Assume95%confidence.