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. 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 , and Za/2 relates to the level of confidence. n = (2/2) E 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 75 seconds. minutes 60 seconds Therefore, the margin of error, in minutes, is E = minutes.