Aone-million-liter pond is contaminated by a chemical pollutant with a concentration of 20 g/l. the source of the pollutant is removed, and pure water is allowed to flow into the pond at a rate of 1900 l/hr. assuming that the pond is thoroughly mixed and drained at a rate of 1900 l/hr, how long does it take to reduce the concentration of the solution in the pond to 30% of the initial value?