Question

An auto mechanic knows the average time it takes to replace a defective car radiator is 70 minutes with a standard deviation of 12 minutes. this average is based on a random sample of 50. which excel statements will construct a 90% confidence interval for the time needed to replace a defective car radiator?
a. =70+1.96*12/sqrt(50), and =70-1.96*12/sqrt(50)
b. =70+1.96*3.46/sqrt(50), and =70-1.96*3.46/sqrt(50)
c. =70+1.645*3.46/sqrt(50), and =70-1.645*3.46/sqrt(50)
d. =70+1.645*12/sqrt(50), and =70-1.645*12/sqrt(50)
e. none of the above.

Answer 1

Given that the average time it takes to replace a defective car radiator is 70 minutes with a standard deviation of 12 minutes.

The 90% confidence interval of a random sample of 50 is given by

$90\% \ C.I.=70\pm1.645 \frac{12}{\sqrt{50}} \\ \\ \Rightarrow 70+1.645 \frac{12}{\sqrt{50}} \ and \ 70-1.645 \frac{12}{\sqrt{50}}$