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 σ = 100. the composition of the bar has been slightly modified, but the modification is not believed to have affected either the normality or the value of σ.
1) The formula used for determining the confidence interval is Sample mean +/- Critical value (or z-score corresponding to 90% confidence) * Standard error of mean 8439 +/- 1.645 * 100/sqrt 25 8439 +/- 1.645 * 20 8439 +/- 32.9 2) The only difference in this case is finding the critical value i.e., z-score corresponding to 92% confidence which is 1.75 approximately Then the confidence interval is 8439 +/- 1.75 * 100/sqrt 25 8439 +/- 1.75 * 20 8439 +/- 35 Lower limit is 8439 - 35 = 8404 The upper limit is 8439 + 35 = 8474
