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A truckload of materials was purchased for $ 36,000. They were sorted out into the following grades whose market rate is shown as under. No, of Units Selling rate per unit Grade X 5,000 $ 4.80 Grade Y 3,000 $ 4.00 Grade Z 2,000 $ 2.00 Find out the purchase rate per unit of each grade of the materials assuming all grades yield at the same rate of profit