Landings and takeoffs at Schiphol, Holland, per month are (in 1,000s) as follows:

26, 19, 27, 30, 18, 17, 21, 28, 18, 26, 19, 20, 23, 18, 25, 29, 30, 26, 24, 22, 31, 18, 30, 19

Assume a random sample of months. Give a 95% confidence interval for the average monthly number of takeoffs and landings.