A 750-liter tank initially contains 450 liters of brine of concentration 0.25 kg/L. Pure water enters the tank at a rate of 50 liters/hour and brine is pumped out of the tank at the same rate. Develop the equation through which the amount of salt may be calculated at any time.

A 750-liter tank initially contains 450 liters of brine of concentration 0.25 kg/L. Pure water enters the tank at a rate of 50 liters/hour and brine is pumped out of the tank at the same rate. Develop the equation through which the amount of salt may be calculated at any time.