The average of any set of n numbers is the sum of those numbers divided by n.
The average of a, b, and c, is (a+b+c)/3.
In the problem you would first find a common denominator (which is 6x).
1/x, 1/2x, and 1/3x would then change to 6/6x, 3/6x, and 2/6x.
After you find a common denominator, you can add up the numbers.
6/6x + 3/3x + 2/6x = 11/6x
You take the sum and divide by the number of numbers in the set.
(11/6x)/3 = 11/18x 🙂