When you add 1 + 4 + 5 + 5 + 5 + 6 + 9 + 10 + 10 + 11 + 12 + 12 + 15 + 15 + 15 = 135. Which is all of the numbers given to you, and since mean is when you add all of the numbers together, and than divide it by the amount of numbers there, you will take that 135 and divide it by 15. 135 / 15 = 9. Which gets you to finally getting an answer of 9. so D. 9 is correct.
