Question

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.

Answer 1

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