The main idea is to set an ODE using \(\frac{dS\left ( t\right ) }{dt}=R_{in}-R_{out}\) where \(R_{in}\) is rate of mass of salt coming into the tank and \(R_{out}\) is rate of mass of salt leaving tank. This gives an ODE to solve for \(S\left ( t\right ) \) using initial conditions which is given. At end, divide by volume of tank to get concentration at time \(t\). See book example at page 54.