Initially 50 pounds of salt is dissolved in a large tank holding 300 gallons of water. a brine solution is pumped into the tank at a rate of 3 gallons per minute, and the well-stirred solution is then pumped out at the same rate. if the concentration of the solution entering is 2 pounds per gallon, determine the amount of salt in the tank at any time. how much salt is present after 50 minutes?