Rolls Royce makes the Trent 900 jet engines used in the new Airbus A380 planes, and needs to control the maximum thrust delivered by the engines. The following are readings related to power for successive engines produced:

121, 122, 121, 125, 123, 121, 129, 123, 122, 122, 120, 121, 119, 118, 121, 125, 139, 150, 121, 122, 120, 123, 127, 123, 128, 129, 122, 120, 128, 120

Aggregate the data in groups of 3, and create a control chart for the process mean. Use the chart to test the assumption that the production process is under control.