3. A spark plug manufacturer claimed that a new manufacturing procedure has changed the mean life of their plugs from 22,100 miles. Assume that the life of the spark plugs follows the normal distribution. A fleet owner purchased a large number of sets. A sample of 18 sets revealed that the mean life was 23,400 miles and the standard deviation was 1,500 miles. Is there enough evidence to substantiate the manufacturer’s claims at the 0.05 significance level?