If Standard Deviation is known, on the basis of results obtained from a random sample of 400 normally distributed items, the 95% confidence interval f

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If Standard Deviation is known, on the basis of results obtained from a random sample of 400 normally distributed items, the 95% confidence interval for μ was found to be 1999.853 and 2000.147. How to find values of the sample mean (ത) and SD?