Tutorial Exercise Customers arrive at a movie theater at the advertised movie time only to find that they have to sit through several previews and pre preview ads before the movie starts Many complain that the time devoted to previews is too long t A preliminary sample conducted by The Wall Street Journal showed that the standard deviation of the amount of time devoted to previews was 5 minutes Use that as a planning value for the standard deviation in answering the following questions a If we want to estimate the population mean time for previews at movie theaters with a margin of error of 75 seconds what sample size should be used Assume 95 confidence b If we want to estimate the population mean time for previews at movie theaters with a margin of error of 1 minute what sample size should be used Assume 95 confidence Step 1 a If we want to estimate the population mean time for previews at movie theaters with a margin of error of 75 seconds what sample size should be used Assume 95 confidence Recall the formula to find a sample size n for a desired margin of error E with a population standard deviation o and 2 2 relates to the level of confidence za 2 E n The sample standard deviation of 5 minutes will be used in place of the population standard deviation However note that the desired margin of error was given to be 75 seconds The standard deviation and margin of error must be in terms of the same units Convert 75 seconds to minutes 1 minute 60 seconds 75 seconds minutes Therefore the margin of error in minutes is E minutes