averages are figured like all averages. add all the ages of all the planes together and divide by the number of planes.
example
5 air planes with ages of 1,1,3, 10,5 =20 divided by 5 = an average age of 4 years old
or
2,3,1,6,6,=18 divided by 5 = 3.6 this goes for how ever many planes or items you want
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