standard deviation?

Question

In the manufacturing process of ball bearings, the mean diameter is 5mm with a standard deviation of 0.002 mm. Each hour a sample of 20 bearings is drawn, measured, and the mean diameter of the sample found. What is the standard deviation o x of the sample

Answer 1

Use Central Limit theorem.

sigma for a sample mean (x-bar)=sigma for the population/ square root of n

So heresigma for x-bar= 0.002/ sqrt(20) = .0004472135956 approximately