A tank is filled with 8 gallon of brine in which 2 lb of salt is dissolved. Brine having 3 lb of salt per gallon enters the tank at 4 gallon per minute, and the well stirred mixture leaves the tank at the same rate. Does the volume of brine in the tank change with time? Find the concentration of the salt (lb/gal) in the tank as a function of time. With the amount of salt in the tank, x(t), the volume of the tank V, the flow rate of brine into the tank of qin and out of the tank qout : dx dt = amount of salt entering the tank - amount of salt leaving the tank = inlet concentration of salt *qin - x(t) V * qout = 3 lb salt/gal *4 gal/min - x(t) V *4 gal/min.