c.
What is the probability that your survey will provide a sample proportion within ± 0.15 of
the population proportion?

Chapter 8 Problem 10 Pg. 349
Q 10.
Playbill magazine reported that the mean annual household income of its readers
is$119,155 (Playbill, January 2006). Assume this estimate of the mean annual income household
income is based on a sample of 80 households, and based on past studies, the population standard
deviation is known to be σ = $30,000.
a.
Develop a 90% confidence interval estimate of the population mean.
1.645

So (119155-1.645*30000/
√
80
, 119155+1.645*30000/
√
80
0 is a confidence interval of
population mean
(113638, 124672) is 90% confidence interval for µ.
b.
Develop a 95% confidence interval estimate of the population mean.
1.96

c.
Develop a 99% confidence interval estimate of the population mean.

d.
Discuss what happens to the width of the confidence interval as the confidence level is
increased. Does this result seem reasonable? Explain.

Chapter 8 Problem 26 Pg. 361
Q 26.
The U.S. Energy Information Administration (US EIA) reported that the average price for
a gallon of regular gasoline is$3.94 (US EIA website, April 6, 2012). The US EIA updates its
estimates of average gas prices on a weekly basis. Assume the standard deviation is $.25 for the

price of a gallon of regular and recommend the appropriate sample size for the US EIA to use if
they wish to report each of the following margins of error at 95% confidence.
a.
The desired margin of error is $.10.

b.
The desired margin of error is $.07.
= 49

c.
The desired margin of error is $.05.