To determine whether they need to hire extra for the holiday season, the managers of a railroad company want to estimate how long, on average, it takes a crew to unload a freight car at the lafayette station. they will calculate an estimator as the sample mean of a random sample of unloading times. the managers want the sample mean to have a standard error of 1.51.5 minutes. the corporate office has provided a standard deviation of 99 minutes to use for calculation purposes. how large should the random sample be to ensure the sample mean has the desired standard error?