Answer:
15 + 16 + 17 + 18 + 19 + 20 + 21 + 22 + 23 + 24 + 25 = 220
220/11 = 20
The answer is 20.
You can find this by adding all of the numbers in the data set together and then dividing it all by numbers of values you added! (The mean is the same as an average.)