Records on a fleet of trucks reveal that the average life of a set of spark plugs is normally distributed with a mean of 22,100 miles. A manufacturer of spark plugs claims that its plugs have an average life equal to 22,100 miles. The fleet owner purchased 18 sets and found that the sample average life was 20,800 miles, the sample standard deviation was 1,500 miles. If significance level = 0.05. What is the conclusion?