Models & Qualitative Behavior · Superchallenge problem

Two $100$-litre tanks are joined by two pipes: one pumps $10$ L/min from tank A to tank B, the other $10$ L/min from B back to A, and both tanks are kept well mixed. Initially A holds $300$ g of dissolved salt and B holds none. After how many minutes does B hold $100$ g of salt? Give an exact answer.