An independent group of food service personnel conducted a survey on tipping practices in a large metropolitan area. They collected information on the percentage of the bill left as a tip for 40 randomly selected bills. The average tip was 12.1% of the bill with a standard deviation of 2.6%. Assume that the tips are approximately normally distributed. Construct an interval to estimate the true average tip (as a percent of the bill) with 90% confidence. Round the endpoints to two decimal places, if necessary.

Considering the following scenario, which method would be most appropriate when calculating the margin of error for the population mean?
σ is known; n=52 ; the population is normally distributed.

The state education commission wants to estimate the fraction of tenth grade students that have reading skills at or below the eighth grade level. In an earlier study, the population proportion was estimated to be 0.16.
How large a sample would be required in order to estimate the fraction of tenth graders reading at or below the eighth grade level at the 85% confidence level with an error of at most 0.03? Round your answer up to the next integer.
Using the data, construct the 80% confidence interval for the population proportion of new car buyers who prefer foreign cars over domestic cars. Round your answers to three decimal places.

An FDA representative randomly selects 8 packages of ground chuck from a grocery store and measures the fat content (as a percent) of each package. Assume that the fat contents have an approximately normal distribution. The resulting measurements are given below.

Fat Contents (%)
19 18 11 17
17 19 12 15

Calculate the sample mean and the sample standard deviation of the fat contents. Round your answers to two decimal places, if necessary.
b. Construct a 95% confidence interval for the true mean fat content of all the packages of ground beef. Round the endpoints to two decimal places, if necessary.

Out of 159 randomly selected adults in the United States who were surveyed, 81 exercise on a regular basis. Construct a 90% confidence interval for the proportion of all adults in the United States who exercise on a regular basis. Round to three decimal places.