A stream of waste water having BOD of 20.0 mg/L discharges water at the rate 1.0 m3/s into river with flow rate 8.0 m3/s and BOD of 5.0 mg/L. Assuming complete and instantaneous mixing, what is the resultant BOD just downstream from the point of discharge?
1
∼ 12.5 mg/L
2
∼ 6.7 mg/L
3
∼ 15.0 mg/L
4
∼ 1.5 mg/L