First, you add them all together, and divide them by the number of quantities there are, so: 9+9+8+11+11+12+12+8= <span>80. Then you divide it by 8 (how many numbers there are) to get 10!</span>
If you are doing an estimate, then you need to look at the numbers to see kinda what they are around. This set of data seems to be revolving around the number 10 because there are 4 above and 4 below. So I would put down 10 for my answer.