To find the mean of a set of numbers, we add up all the numbers and divide by the total count of the numbers.
Adding up the numbers, we get:
6 + 9 + 11 + 15 + 20 = 61
There are 5 numbers in the set, so to find the mean we divide 61 by 5:
61/5 = 12.2
Therefore, the mean of 6, 9, 11, 15, and 20 is 12.2.