Long Last Tire Company, as part of its inspection process, tests its tires for tread wear under simulated road conditions. Twenty samples of three tires each were selected from different shifts over the last month of operation. The tread wear is reported below in hundredths of an inch.

a. Determine the control limits for the mean and the range.
b. Plot the control limits for the mean tread wear and the range.
c. Are there any points on the mean or the range chart that are €œout of control€? Comment on thechart.

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Outside Diameter (millimeters) Time 8:00 8:30 9:00 9:30 10:00 10:30 87.1 86.9 87.5 86.0 87.1 88.0 87.3 88.5 88.4 88.0 87.1 86.2 87.9 87.6 86.9 87.2 87.1 87.4 87.0 87.5 87.6 87.6 87.1 87.3 87.0 87.4 88.2 87.1 87.1 87.8