A tank with a capacity of 100 gallons initially contains 50

A tank with a capacity of 100 gallons initially contains 50 gallons of water with 10 pounds of salt in solution. Fresh water enters at a rate of 2 gallons per minute and a well-stirred mixture is pumped out at the rate of 1 gallon per minute. Compute the amount of salt (in pounds) in the tank at the first moment when the tank is filled.