8. The standard deviation of test scores on a certain achievement test is11.2.A random sample of60scores on this test had a mean of71.4.Based on this sample, find a95%confidence interval for the true mean of all scores. Then give its lower limit and upper limit.

Carry your intermediate computations to at least three decimal places. Round your answers to one decimal place. (If necessary, consult alist of formulas.)

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9. Managers at an automobile manufacturing plant would like to estimate the mean completion time of an assembly line operation,. The managers plan to choose a random sample of completion times and estimatevia the sample. Assuming that the standard deviation of the population of completion times is10.2 minutes, what is the minimum sample size needed for the managers to be99%confident that their estimate is within2.2minutes of?

Carry your intermediate computations to at least three decimal places. Write your answer as a whole number (and make sure that it is the minimum whole number that satisfies the requirements).

(If necessary, consult alist of formulas.)

10. A union of restaurant and foodservice workers would like to estimate the mean hourly wage,, of food service workers in the U.S. The union will choose a random sample of wages and then estimate

using the mean of the sample. What is the minimum sample size needed in order for the union to be99% confident that its estimate is within$0.45of? Suppose that the standard deviation of wages of food service workers in the U.S. is about$

2.20 . Carry your intermediate computations to at least three decimal places. Write your answer as a whole number (and make sure that it is the minimum whole number that satisfies the requirements).

14. A researcher collected sample data for9women ages18 to24. The sample had a mean serum cholesterol level (measured in mg/100 mL) of193, with a standard deviation of9.5. Assuming that serum cholesterol levels for women ages18 to24are normally distributed, find a95% confidence interval for the mean serum cholesterol level of all women in this age group. Then find the lower limit and upper limit of the95%confidence interval.Carry your intermediate computations to at least three decimal places. Round your answers to one decimal place. (If necessary, consult alist of formulas.)

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17. From a large number of actuarial exam scores, a random sample of350 scores is selected, and it is found that255 of these350 are passing scores. Based on this sample, find a99%confidence interval for theproportion of all scores that are passing. Then find the lower limit and upper limit of the99%confidence interval.Carry your intermediate computations to at least three decimal places. Round your answers to two decimal places. (If necessary, consult alist of formulas.

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18. Suppose that255out of a random sample of350letters mailed in the United States were delivered the day after they were mailed. Based on this, compute a99%confidence interval for theproportion of all letters mailed in the United States that were delivered the day after they were mailed. Then find the lower limit and upper limit of the99%confidence interval.

Carry your intermediate computations to at least three decimal places. Round your answers to two decimal places.\

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7. The lifetime of a certain brand of battery is known to have a standard deviation of23 hours. Suppose that a random sample of70 such batteries has a mean lifetime of36.6hours. Based on this sample, find a90% confidence interval for the true mean lifetime of all batteries of this brand. Then give its lower limit and upper limit.

Carry your intermediate computations to at least three decimal places. Round your answers to one decimal place.

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upper limit

8. The standard deviation of test scores on a certain achievement test is11.2.A random sample of60scores on this test had a mean of71.4.Based on this sample, find a95% confidence interval for the true mean of all scores. Then give its lower limit and upper limit. Carry your intermediate computations to at least three decimal places. Round your answers to one decimal place.

lower limit:

upper limit::