The answer is not 45!

If a plane has 60 passengers on it, it's more likely that you'll be one of those passengers than a passenger on a 30-passenger airplane.  What's needed is a weighted average of the number of passengers on each plane, weighted by the probability that you're there to observe it, which is proportional to 1 over the number of passengers.  So:

Average = [(1/60)*5*60+(1/30)*5*30]/[(1/60)*5+(1/30)*5] = 40

(the term in the denominator is the sum of the weights.)

Don't believe me?  Suppose that 15 planes take off, ten with 30 passengers, and five with 60 passengers.  For these 15 flights, the average number of passengers is 40.  However, if you asked each passenger how many people were on his/her plane, and averaged all the answers, you'd get 300 people answering 30, and 300 people answering 60.  The average answer is therefore 45.  But the average number of passengers is 40.  The same would be true if you selected 10 passengers at random; on average you'd get five answers of 30 and five answers of 60, which would average to 45.  If you take ten trips on an airline, that's like ten random samples of the number of passengers, so it's the same as selecting ten people at random and asking them how many passengers were on their plane.

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