A farmer will fence three sides of a rectangular pen adjacent to a straight river, leaving the side along the river unfenced. The side parallel to the river uses reinforced fencing costing \18permeter,whilethetwoperpendicularsidesusestandardfencingcosting10 per meter each. The farmer can spend at most \720onfencing.Letwdenotethewidthofthepenperpendiculartotheriver.Determinethevalueofw$ (in meters) that maximizes the enclosed area.