2a. A medical devices company wants to know the number of hours its MRI machines are used per day. A previous study found a standard deviation of two hours. How my MRI machines must the company study in order to have a margin of error of + 0.4 hours when calculating a 95% condence interval? 21). How many mid-level managers need to be selected to have 85% condence in estimating the population proportion of mid-level managers who cite stress as a cause of absence? Margin of error is H.075. Previous study found proportion of 60%