Between 9 AM and 10 AM one hundred airplanes depart from Newark Airport. Ten of the airplanes were delayed by an hour each. Of the remaining airplanes, half left on time and half were delayed by an average of 20 minutes. To the nearest whole minute, how long was the average flight delay?
A. 15.
B. 18.
C. 27.
D. 30.
E. 40.

E. 40

Step-by-step explanation:

the average is the relationship between the sum of data and the number of data, in this case you have 100 planes of which 10 left with an hour delay (60min) and the rest (90 planes) half (45 planes) planes they left on time and the other half (45 aircraft) with 20 min delay , so average is equal to the sum of the data between the number of data .

P=(60+20)/2 = 40

In this case, only the airplanes whose events differ with the fact of leaving on time