Systems of Differential Equations · Medium problem

Two tanks each hold $100$ L of brine. Well-mixed solution is pumped from tank A to tank B at $10$ L/min and from tank B back to tank A at $10$ L/min; nothing else enters or leaves. Initially tank A holds $20$ kg of salt and tank B holds none. How many kilograms of salt are in tank A at time $t=5\ln 2$ minutes?