The useful life a specific electrical product in years is known to be normally distributed with variance= 2 years. A random sample of 25 products has a mean of useful

life of equals to 5.5 years. (a) Construct a 95% two-sided confidence interval on the mean useful life. (b) Construct a 90% two-sided confidence bound on the mean useful life. (c) Construct a 90% lower confidence bound on the mean useful life. (d) Compare the width of the two intervals in (a) and (b), and explain why they different?are