9.2

21.

  1. Flawed, here the population mean changes rather than the interval.
  2. Correct.
  3. Flawed, the interpration focuses on the individuals rather than the mean.
  4. Flawed, the interpretation focuses on the adults in Idaho rather than the mean number of hours worked by adult Americans.

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

25. To narrow the confidence interval, increase the sample size and decrease the level of confidence.

31.

  1. Point estimate x hat = 4.893.
  2. Lower bound= 4.690, upper bound= 5.096. We are 95% confident the mean pH of rain water in Tucker County is between 4.690 and 5.096.
  3. Lower bound= 4.607, upper bound= 5.179. We are 99% confident that the mean pH of rain water in Tucker County is between 4.607 and 5.179.
  4. Level of confidence and margin of error increase simultaneously.

32.

  1. Point estimate x hat = 83.35.
  2. We are 95% confident that the mean tax paid for a three-day business trip is between 73.05 and 93.65.
  3. Reduce the sample size.

33.

  1. Lower bound= 1035.8, upper bound= 2477.2. We are 95% confident that the mean repair cost for a low impact collision involving mini or micro vehicles is between 1035.8 and 2477.2
  2. With less variability with the data, the 95% confidence interval would be more narrow.