In order to fairly set flat rates for auto mechanics, a shop foreman needs to estimate the average time it takes to replace a fuel pump in a car. Assume that the standard deviation of all times is 30 minutes.
a. Determine the minimum sample size he must select if he wants to be 90% confident that the true average time is within 10 minutes of the sample average?
b. Repeat part (a) using a 95% confidence interval.