The definition of mean, is to add up all the numbers and divide it by how many numbers there are. So, in this case the mean would be 1+1+1+2+2+2+2+3+3+4+4+4+4+5+5+6+7+7+8+8+9 = 86. You would then take 86 and divide that by 21, and you would get 4.0 The definition of median states to find the middle of the numbers. So the median would be 4. So you would just take 4 - 4 = 0.
