A perishable product is purchased by a retailer for 5 and sold for 9. Based on past experience, it is estimated that 4 of the time 10 items can be sold, k of the time 1 1 items are sold, and A of the time 12 items are sold. There is also a A probability that a strike will occur and no items can be sold at all. If an item is not sold, the retailer will lose 5. Find the number of items he should purchase in order to maximize profit.