Ex.11

10. The average life of a sample of 10 tires of a certain brand was 28,406 miles. If it is known that the lifetimes of such tires are normally dis- 8 tributed with a standard deviation of 3300 miles, determine a 95 percent confidence interval estimate of the mean life. C 11. For Prob. 10, how large a sample would be needed to obtain a 99 percent confidence interval estimator of smaller size than the interval obtained in the problem? 12. A pilot study has revealed that the standard deviation of workers