What are the 99.7% upper and lower control limits

A company is producing cylindrical blocks for a children's toy. The average diameter of a block is 3.8 inches. A sample of 30 blocks is inspected and the diameters are measured. The standard deviation is estimated to be 0.04 inches.

What are the 99.7% lower and upper control limits for the x-bar chart?.. Answer should be two decimal places

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