The lifetime of a certain brand of battery is known to have a standard deviation of 9 hours. Suppose that a random sample of 90 such batteries has a mean lifetime of 37.9 hours. Based on this sample, find a 99% confidence interval for the true mean lifetime of all batteries of this brand. Then complete the table below. (SEE attachment for table that needs answers)What is the lower limit of the confidence interval?What is the upper limit of the confidence interval?

Solution 

s=9

n=90

x=38.9

alpha=0.10

Za/2 = Z0.05 = 1.645

confidence interval:

x +/- [Za/2 * (s/sqrt(n))]

38 +/- 1.56

= (36.44, 39.56)

Lower limit = 37.09,

upper limit = 40.70