4) (15 points) A wastewater treatment plant discharges 2.0 m³/s of effluent having an ultimate BOD of 40.0 mg/L into a stream flowing at 15.0 m³/s. Just upstream from the

discharge point, the stream has an ultimate BOD of 3.5 mg/L. Assuming complete and instantaneous mixing, find the ultimate BOD of the mixture of wastewater and river just downstream from the outfall.