that a given alternator will fail on a 1-hour flight is .02. What is the probability that (a) both will
fail? (b) Neither will fail? (c) One or the other will fail? Show all steps carefully.
5.70 The probability is 1 in 4,000,000 that a single auto trip in the United States will result in a
fatality. Over a lifetime, an average U.S. driver takes 50,000 trips. (a) What is the probability of
a fatal accident over a lifetime? Explain your reasoning carefully. Hint: Assume independent
events. Why might the assumption of independence be violated? (b) Why might a driver be
tempted not to use a seat belt “just on this trip”?
8.46 A random sample of 10 miniature Tootsie Rolls was taken from a bag. Each piece was weighed on
a very accurate scale. The results in grams were
3.087 3.131 3.241 3.241 3.270 3.353 3.400 3.411 3.437 3.477
(a) Construct a 90 percent confidence interval for the true mean weight. (b) What sample size
would be necessary to estimate the true weight with an error of } 0.03 grams with 90 percent confidence?
(c) Discuss the factors which might cause variation in the weight of Tootsie Rolls during
manufacture. (Data are from a project by MBA student Henry Scussel.)
8.62 In 1992, the FAA conducted 86,991 pre-employment drug tests on job applicants who were to be
engaged in safety and security-related jobs, and found that 1,143 were positive. (a) Construct a
95 percent confidence interval for the population proportion of positive drug tests. (b) Why is the
normality assumption not a problem, despite the very small value of p? (Data are from Flying 120,
no. 11 [November 1993], p. 31.)
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