40. If X1, . . . , Xn is a sample from a normal population, explain how to obtain a 100(1 − α) percent confidence interval for the population variance σ2 when the population mean μ is known. Explain in what sense knowledge of μ improves the interval estimator compared with when it is unknown.

Repeat Problem 38 if it is known that the mean burning time is 53.6 seconds.