A Quality Analyst wants to construct a control chart for determining whether three machines, all producing the same product, are under control with regard to a particular quality variable. Accordingly, he sampled four units of output from each machine, with the following results : Machine #1 measurements [23, 18, 15, 20]; Machine #2 measurements [ 24, 13, 13, 15]; Machine #3 measurements [ 14, 18, 22, 15]. Using the factors for three sigma control limits, what are x-bar chart upper and lower control limits?