Can any one help me outA new Firestone tire is guaranteed to last for 40,000 miles. The actual mean life of the tires is 47,000 miles with a standard deviation of 4,000 miles.

a) What is the probability that a tire will not last for 40,000 miles?
b) The Ford Motor Company uses Firestone Tires and is concerned about the performance of the tires. They conduct a study and find that out of the 120 tires they tested, the mean life was only 46,500 miles. Should the Ford Motor Company be concerned about Firestone's honesty about its tires? Provide a statistical justification for your response.

Question

Can any one help me outA new Firestone tire is guaranteed to last for 40,000 miles. The actual mean life of the tires is 47,000 miles with a standard deviation of 4,000 miles. 

a) What is the probability that a tire will not last for 40,000 miles? 
b) The Ford Motor Company uses Firestone Tires and is concerned about the performance of the tires. They conduct a study and find that out of the 120 tires they tested, the mean life was only 46,500 miles. Should the Ford Motor Company be concerned about Firestone's honesty about its tires? Provide a statistical justification for your response.

amistre64 · Answer

z = (x-mean)/sd

and for a given sample size, n, we want to determine z*sqrt(n)