Suppose that a large mixing tank initially holds 1200 L of water in which 25 kg of salt have been dissolved. Another brine solution is pumped into the tank at a rate of 10 L/min, and when the solution is well stirred, it is then pumped out at a slower rate of 7.5 L/min. If the concentration of the solution entering is 0.25 kg/L, determine a differential equation for the amount of salt A(t) in the tank at time t > 0. (Use A for A(t).)