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 R-chart (control chart).

Observed customer service time (seconds) teller	Fred	Ginger	Laura	Tom
Mon.	80	120	185	140
Wed.	155	130	135	120
Fri.	120	160	105	80