In May, four bank tellers were observed as they served a customer at the main counter. The time required to complete the service was measured on each of three days and the results are shown in the table below. Using the table below, determine the upper and lower control limits for an X-chart (control chart). observed customer service time (seconds) teller Mon. wed. fri. Fred 80 155 120 Ginger 120 130 160 Laura 185 135 105 Tom 140 120 80