All 80 rooms in a motel will be rented each night if the manager charges $40 or less per room. If he charges $ (40 + x) per room, then 2x rooms will remain vacant. If each rented room costs the manager $10 per day and each unrented room $2 per day in overhead cost, how much should the manager charge per room to maximize his daily profit? Note that the profit function is positive when 0 S x S 38