in class we looked at a mixing problem in which the volume of fluid remained constant and saw that such problem gives rise to a separable differentiable equation. if the rates of flow into and out of the system are different, then the volume is not constant and the resulting differential equation is linear but not separable. a tank contains 100 l of water. a solution with a salt concentration of 0.4 kg/l is added at a rate of 5 l/min. the solution is kept mixed and is drained from the tank at a rate of 3 l/min. if y(t) is the amount of salt (in kilograms) after t minutes, show that y satisies the differential equation