Question: The manufacturer of a steel-belted radial tire claims that the mean mileage the tire can be driven before the tread wears out is 80,000 kilometres. The standard deviation of tread life is 8,000 km. A truck company purchased 49 of these tires and found a mean tread life of only 79,200. On the basis of this data, test the null hypothesis that the mean tread life is at least 80,000 km. against the alternative that the mean life is less than 80,000 km. Use a 5% level of significance.