1 . A civil engineer is analyzing the compressive strength of concrete. Compressive strength is approx- imately normally distributed with a variance o' = 1000 (psi). A random sample of 12 specimens has a mean compressive strength of x = 3250 psi. (a) Construct a 95% two-sided confidence interval on mean compressive strength. (b) Construct a 99% two-sided confidence interval on mean compressive strength. Compare the width of this confidence interval with the width of the one found in part (a). 2 Suppose that in Exercise 10-41 it is desired to estimate the compressive strength with an error that is less than 15 psi. What sample size is required