A magazine stall purchases a fashion magazine at $2.5 at the beginning of each week and sell them at $8.5 during the week. Any unsold copies of this magazine at the end of the week can be salvaged at $0.5. The weekly demand for this magazine is Normal* with mean and stanrard deviation of 8. How many of this magazines should the stall buy in the beginning of each week so to maximize its expected profits?