Suppose a large labor union wishes to estimate the mean number of hours per month a union member is absent from work. The union decides to sample 316 of its members at random and monitor the working time of each of them for 1 month. At the end of the month, the total number of hours absent from work is recorded for each employee. If the mean of the sample is 9.9 hours and the population standard deviation is 2.4 hours, find a 90% confidence interval for the true mean number of hours a union member is absent per month.
9.9 ± 1.43
9.9 ± .012
9.9 ± .122
9.9 ± .222