The formulae to be used in this question are simple. You are given the total cost and quantity, and thus average total cost is simply Total Cost / Quantity = 50. From the given average variable cost, we can get total varaible cost (TVC) = AVC x Quantity = 150. From TVC and TC, we get TFC as TC - TVC = 350. From TFC, dividing it by quantity, we get AFC = 35.