Question: A spark plug manufacturer claims that the lives of its spark plugs are normally distributed with a mean of 60000 miles and standard deviation of 4000 miles.

(a) A random sample of 16 plug had an average life of 58250 miles. If the manufacturers claim is correct (that mean is 60000), what is the probability of finding a sample mean of 58250 or less?

(b) Based on your answer to part (a), what will be your reaction about the manufacturer’s claim? Discuss.