Module for Mixing Solute

Numerical Methods for O. D. E. using Mathematica. (c) John H. Mathews, 2003

Example 1.  Initially, at time t=0, a tank contains a mixture: 50 g of salt dissolved in 10 L of water.  A solution containing 10 g/L of salt is pumped into the tank at a rate of 5 L/min, and the well-stirred mixture flows out at a rate of 4 L/min.
(a)  Write down the differential equation for the amount of salt as a function of time.
(b)  Solve the differential equation.
(c)  Find the solution that has the specified initial condition at time t=0.