Able trucking company has a job moving 21 tons of sand. the company has 8 drivers in the company and 2 types of trucks. one type of truck can haul 5 tons of sand and the other type of truck can haul 3 tons. insurance requirements make it necessary for trucks hauling 5 tons of sand to have two drivers in the cab during the operation. three ton trucks require only one driver. using all available drivers, how many trucks of each size will be needed to move the sand in one trip?