6. On the basis of extensive tests, the yield point of a particular type of mild steel reinforcing bar is known to be normally distributed with s100. The composition of the bar has been slightly modi ed, but the modi cation is not believed to have affected either the normality or the value of s.

a. Assuming this to be the case, if a sample of 25 modi ed bars resulted in a sample average yield point of 8439 lb, compute a 90% CI for the true average yield point of the modi ed bar.

b. How would you modify the interval in part

(a) to obtain a con dence level of 92%?