In our problem, the level of water remains the same, as we have and flowing in and flowing out.
If we call the salt level , then we have
To find , we subtract the rate of salt flowing out from the rate flowing in:
The rate flowing in is
Since the fluid is well-mixed, the rate flowing out will be in proportion to as the current salt level is to the entire resevoir. Therefore,
For , we obtain
Now, all that remains is to solve the differential equation using the initial conditions given.