More Practice Name________________________________
1.
The president of a large university wishes to estimate the average age of the students presently enrolled.
From past studies, the standard deviation is known to be 2 years. A sample of 50 students is selected, and the mean is found to be 23.2 years. Find the 90% confidence interval of the population mean.
2.
In a certain state, a survey of 500 workers showed that 23% belonged to a union. Find the 95% confidence interval of the true proportion of workers who belong to a union.
3.
A health care professional wishes to estimate the birth weights of infants. How large a sample must she select if she desires to be 90% confident that the true mean is within 6 ounces of the sample mean? The standard deviation of the birth weights is known to be 8 ounces.
4.
A company that installs swimming pools for residential customers sells a pool package to 18% of the customers its representatives visit. If the sales reps visit 180 customers, find the probability of their making at least 40 sales.

5.
A sample of 20 tuna showed that they swim an average of 8.6 miles per hour with a standard deviation of
1.6. Find the 90% confidence interval of the true mean.
6.
A researcher wishes to be 92% confident that her estimate of the true proportion of individuals who travel overseas is within 0.04 of the true proportion. Find the sample size necessary.
7.
The average lifetime of irons is 9 months. The standard deviation was 3.1 months. If 24 irons are selected, find the probability that the mean lifetime is between 8 and 10 months.
8.
What does the 90% confidence level mean?