I think this is the right ODE setting and its solution:
Y’= inflow of salt – outflow of salt
Y’= 3 – 0.01y
Y’= -.01 (y –300)
dy/dt = –.01 (y–300) dt
dy/(y–300)= –.01 dt
Integrate both sides yields:
∫dy/(y–300)= ∫ –.01 dt
ln Iy-300I= –.01t + c
Take exponential for both sides:
Y–300=...