In a test measuring the life span of a certain brand of tire, 100 tires are tested. The results showed an average lifetime of 50,000 miles, with a standard deviation of 5000 miles. One hundred thousand of these tires have been sold and are on the road.
(a) After what mileage would you expect 10% of the tires to have worn out?
(b) How many tires are expected to wear out between 60,000 and 70,000 miles?
(c) How many tires are expected to have a life of less than 20,000 miles?
(d) What are your major assumptions in these calculations?