Hello, I didn't know where to put my question, but I think here is the best section for it. http://im60.gulfup.com/apkrpJ.png [Broken] The problem isn't that I can't solve it, I actually did but I don't understand the concept !! -- I don't remember anything from my high school stat and I didn't do college stat yet. to be more specific in my question, what does the squire of the deviation mean? and how taking the sum of them give me the result I want?