EXERCISE 4.18 Daily Express Airlines is concerned about the occupancy levels and the on-time performance of their flights. (a) To estimate the occupancy level of their flights, the airline has randomly sampled 20 of its flights. The observed sample mean in the sample is 8.1 unoccupied seats per flight and the observed sample standard deviation is 3.9 unoccupied seats per flight. Construct a 99% confidence interval for the mean number of unoccu pied seats per flight