The median is usually preferred in these situations because the value of the mean can be distorted by the outliers. However, it will depend on how influential the outliers are. If they do not significantly distort the mean, using the mean as the measure of central tendency will usually be preferred.
When you solve that you get 30.825 because you divided 123.3 which was your dimensions times 9 and if you divide that by 4 you get 30.825 . Let me know if that is correct if not I will find new solution!
