9.2

21.

  1. This interpretation is wrong because it uses the “probability” not the confidence level.
  2. This interpretation is correct because it defines the confidence level for the interval.
  3. This interpretation is wrong because it does not use confidence level.
  4. This interpretation is wrong because it does not pertain to Americans.

23. We are 90% confident that the mean drive through service time at Taco Bell is between 161.5 and 164.7 seconds.

25. Increasing the precision of the interval can be done by decreasing the confidence interval level and increasing the sample size.

31.

  1. 4.893
  2. We are 95% confident that the mean pH of rainwater in Tucker County is between 4.690 and 5.096.
  3. We are 99% confident that the mean oH of rainwater in Tucker County is between 4.607 and 5.179.
  4. As the confidence level increases, the interval increases. This is because there is a greater confidence if the interval is larger.

32.

  1. $6.58
  2. We are 95% confident that the interval for the mean tax paid is between -3.01 and 16.17.
  3. To increase precision of the interval, decrease the confidence interval and increase the sample size.

33.

  1. We are 95% confident that the population mean cost of repair is between $1035.8 and $2477.3.
  2. The confidence interval would be narrower as there is less data.