Question: A survey of 200 customers passing though the check-out line of Willy's Supermarket revealed a mean cheek-out time of l 50 seconds and a standard deviation of 60 seconds. For Sally's Supermarket, the mean and standard deviation computed from a sample of 120 shoppers were 125 and 35 seconds, respectively. Calculate a 95% confidence interval for the difference in the mean waiting time at the two supermarkets.