The lifetimes of batteries produced by a firm are known to be normally distributed with a mean of 100 hours and a standard deviation of 10 hours. What is the probability a battery will last between 110 and 120 hours?

Answer 1

Z = (score-mean)/SD

Find table in the back of your statistics text labeled something like "areas under normal distribution" to find the proportion related to the Z scores.

Answer 2

The lifetime (in years) of 9 automobile batteries of a certain brand are: 2.4, 1.9, 2.0, 2.1, 1.8, 2.3, 2.1, 2.3, 1.7. At alpha O.05 level of significance, test if the mean battery life is less than 25 years.
State the null and alternative hypothesis.
State your conclusion for your hypothesis
Give a conclusion for the confidence interval