The usual way of finding the mean within a dataset is to add the data values together and divide by the number of values in the set. An easier way is to remove a bias (in this case 200 as an easy number) first and take off the bias from each value before adding: -2, 3, 41, 55, 11, -1, -2. Added together we get 105, divided by 7=15. Add back the bias=215, so the mean is 215.