8-20 (9-21)
                8-23 (9-24)
                8-25 (Problem A: Suppose that in problem 9-24 we wanted to be 95% confident that the error in estimating
                                            the mean life is less than 0.8 hours. What sample size should be used?)
 

                Notes: 1.the numbers in brackets are for the new edition of the textbook
                           2. for problem 9-24 from the new book, solve (a) through (d) from the book
                               and, in addition:
                              (e) construct a 95% two-sided confidence interval on the mean life
                              (f) construct a 95% lower-confidence interval on the mean life