For 50 airplanes that arrived late at an airport during a week, the time by which they were late was observed. In the following table, x denotes the time (in minutes) by which an airplane was late, and f denotes the number of airplanes.
x f
0 to less than 20......... 1 4
20 to less than 40......... 18
40 to less than 60......... 9
60 to less than 80......... 5
80 to less than 10......... 0 4
Find the mean, variance, and standard deviation.