Question 43749
The mean or average of a set of numbers is found by adding of the numbers then dividing the total by the number of numbrs you added.

In your problem, you will add up the given set of numbers:

13+15+18+18+18+19+22+26+28+33 = 210

Now you will divide this total (210) by how many numbers there are, and that's 10.  So:

{{{210/10 = 21}}}

The mean is 21