15. A tank contains 500 liters of brine with 250 grams of salt in the solution....

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15. A tank contains 500 liters of brine with 250 grams of salt in the solution. Pure water runs into the tank at the rate of 15 liters/minute and the mixture, constantly stirred, is drained out at the same rate. How much salt remains in the tank after 3 hours? 16. A cistern contains 50 gallons of brine with 10 pounds of salt. Another brine solution containing 2 pounds of salt per gallon is pumped into the cistern at 3 gallons per minute and the mixture runs out at the same rate. If the cistern is constantly stirred, find the amount of salt in the cistern after t minutes. 17. A room is 20 ft long, 30 ft wide and 12 ft high. It has been found it is con- taminated with carbon dioxide whose strength is 0.09%. Find the percentage. of carbon dioxide in the room after 18 minutes if air containing only 0.02% is pumped into the room at 2000 cubic ft/min. 18. A tank contains 100 gallons of brine with 300 pounds of salt. It is desired to reduce the amount of salt. To do this, 3 gallons of fresh water are pumped into the tank each minute. At the same time, an outlet value drains the tank at the same rate. When will the tank contain 200 pounds of salt? 19. An aquarium contains 10 gallons of polluted water. To clean the aquarium, fresh water is pumped in at 5 gallons per minute and drained at the same rate. How long will it take to reduce the pollutants to half of its initial level? 15. A tank contains 500 liters of brine with 250 grams of salt in the solution. Pure water runs into the tank at the rate of 15 liters/minute and the mixture, constantly stirred, is drained out at the same rate. How much salt remains in the tank after 3 hours? 16. A cistern contains 50 gallons of brine with 10 pounds of salt. Another brine solution containing 2 pounds of salt per gallon is pumped into the cistern at 3 gallons per minute and the mixture runs out at the same rate. If the cistern is constantly stirred, find the amount of salt in the cistern after t minutes. 17. A room is 20 ft long, 30 ft wide and 12 ft high. It has been found it is con- taminated with carbon dioxide whose strength is 0.09%. Find the percentage. of carbon dioxide in the room after 18 minutes if air containing only 0.02% is pumped into the room at 2000 cubic ft/min. 18. A tank contains 100 gallons of brine with 300 pounds of salt. It is desired to reduce the amount of salt. To do this, 3 gallons of fresh water are pumped into the tank each minute. At the same time, an outlet value drains the tank at the same rate. When will the tank contain 200 pounds of salt? 19. An aquarium contains 10 gallons of polluted water. To clean the aquarium, fresh water is pumped in at 5 gallons per minute and drained at the same rate. How long will it take to reduce the pollutants to half of its initial level?