Items produced by a manufacturing process are supposed to weigh 90 grams. The manufacturing process, however, that there is variability in the items produced and they do not all wight exactly 90 grams. The distribution of weights can be approximated by a normal distribution with mean u = 90 grams nd SD of (sd) = 2 grams. Monitors periodically inspect items produced by weighing a random sample of four items and computing the weight of the four. These averages are plotted on an X bar control chart.

The natural tolerances of individual items are produced by this process are.

The answer is 90 +/- 6 grams. But how do I go about getting this?

Thanks for the help in advance.