9.2

21.

(a)This statement is wrong. Because this interpretation implies that the population mean changes rather than the interval. (b) This statement is correct. Because this interval has been provided for the mean. (c) This statement is wrong. Because this interpretation applied about the individuals rather than the mean. (d)This statement is wrong. Because the interpretation shoud be the mean numbers of hours worked by adult americans, not about adults in the Idaho.

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

25. 1. Increase the length or range of the sample size. 2. Decrease the confidence level.

31.

(a)mean= 58.71/12=4.893 (b)s=0.319 t0.025=2.201 lower bound: 4.893-2.201x0.319/ square root 12= 4.690 upper bound: 4.893+2.201x0.318/square root 12=5.096 We are 95% confident that the mean ph of rain water in turkey county, West Virginia is between 4.690-5.096. (c) t0.005=3.106 lower bound: 4.893-3.106x0.319/square root 12=4.607 upper bound: 4.893+3.106x0.319/ square root 12=5.179 We are 99% confident that the mean ph of rain water in turkey county, West Virginia is between 4.607 and 5.179. (d) The margin of error will increase as the increase of confidence level. 32.

  1. mean=83.349
  2. t0.025=2.365 s=12.32 lower bound: 83.349-2.365x12.32/ square root 8=73.05 upper bound: 83.349+2.365x12.32/ square root 8=93.65 We are 95% confident that the mean tax paid for a three-day business trip will between 73.05 and 93.65
  3. Just directly decrease the confidence level.

33.

  1. mean=17565/10=1756.5 s=1007.5 t0.025=2.262 lower bound: 1756.5-2.262x1007.5/square root 10=1035.8 upper bound:1756.5+2.262x1007.5/square root 10=2477.2 We are 95% confident that the mean repair cost for a low-impact collision involving mini-and micro-vehicles is bwteen $1035.8 and $ 2477.2.
  2. The confidence interval would likely be narrower because there is less variability in the data.Most of the vehicle has been removed.