A tank initially contains 100 gallons of sugar water having a sugar con- centration of 6 pounds per gallon. A sugar solution with a concentration of 2 pounds per gallon is poured into the tank at a rate of 5 gal per minute and is thoroughly mixed at the same time. A drain is opened at the bottom of the tank so that the thoroughly mixed sugar water is leaving the tank at the same rate of 5 gal per minute. How many pounds of sugar are in solution in the tank t minutes after the flow begins?