Mass enters an open system with one inlet and one exit at a constant rate of 50

Question:

Mass enters an open system with one inlet and one exit at a constant rate of 50 kg/min. At the exit, the mass flow rate is 60 kg/min. If the system initially contains 1000 kg of working fluid, determine
(a) dm/dt treating the tank as a system.
(b) The time when the system mass becomes 500 kg.