Assume all samples are from a normal distribution.

Example 1.  A random sample of 50 four-year-olds attending day care centers provided a yearly tuition average of $4500 and the population standard deviation of $630. Find the 90% confidence interval of the true mean.

Example 2.  A random sample of 36 cars in the drive-thru of a popular fast food restaurant revealed an average bill of $18.21 per car. The population standard deviation is $5.92. Estimate the mean bill for all cars from the drive-thru with 98% con­fidence.

Example 3.  A sociologist found that in a random sample of 50 retired men, the average number of jobs they had during their lifetimes was 7.2. The population standard deviation is 2.1.  Find the 95% confidence interval.