Lake Erie has a water volume of about 450 km³ and a flow rate (in and out) of about 175 km² per year. If at some instant the lake has pollution concentration p = 0.04%, how long, approximately, will it take to decrease it to p/2, assuming that the inflow is much cleaner, say, it has pollution concentration p/4, and the mixture is uniform (an assumption that is only imperfectly true)? First guess.