GRE Math subject test #61 Differential Equations Can someone give me a hint on how to find the grams of salt leaving the tank? 
A tank initially contains a salt solution of 3 grams of salt dissolved in $100$ liters of water. A salt solution containing 0.02 grams of salt per liter of water is being sprayed at a rate on 4 liters per minute in the tank. If the sprayed solution is instantaneously mixed and flows out at a rate of 4 liters per minute. How many grams of salt are in the tank after $100$ minutes?
I know that salt is being added at a rate of $0.08$ per minute but I do not know at what rate it is leaving the tank. My friend told me it is equal to $\frac {4x}  {100}$ where x is the number of grams salt present in the tank.
I have no idea where the $100$ came from? Are we not adding water and salt continuously such that the liters of water change? I know how to solve the question if I understand the rate at which salt is leaving. 
Thank you in advance

While there is solution being added to the tank at a rate of $4$ liters per minute, the tank is also being drained at a rate of $4$ liters per minute, so the volume stays constant at $100$ liters, throughout the process.
