Calculating Average with variable range

Hello,

I am trying to come up with a way, without going into VBA or macros, to calculate averages with a variable #. Attached is a list of weeks(Col. A) with qty(Col. B). I would like to figure the average in Col. C of corresponding week, back X(D4) weeks. So for example, in week 10, if I entered "10" in D4, I would like to figure a 10 week average. If D4=5, then I would like to figure a 5 week avg, etc...