Mean = it's really just another word for average. You add all the values up together, and divide the sum by the number of values there are. So here :
9 + 13 + 9 + 10 + 10 + 9 + 10 + 10 + 11 + 9 = 100
And now, you count how many values there are ^ up there. There are 10 values (just count how many numbers there are) and then you have to divide 100 by 10.
