I figure since you're asking about X's you want an algebra equation, don't worry I'll keep it easy.Anyways, to roughly answer your question-
W * D - (W * J) + 8 = X amount of times I went skiing this season
The variable W being the number of weeks that I actually went out to my resort this season.The variable D being the number of days on average I went to my hill in a week that I went out at all.The variable J the times I went out to the hill only to clock in and work in the cafeteria each week.Plus 8 for the eight other times I went skiing after season when the hill was actually closed.
W = 14D = 5.5J = 2.5
Just substitute for W, D, & J.