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Twenty million gallons per day of wastewater with a DO of 2.00mg/L is discharged into a river with a DO of 7.00mg/L. If the flowrate of the river is 150106gal/d and saturation value of dissolved oxygen is 9.17mg/L, what is the DO(mg/L) after tompete mixing of the two flows? 9.17mg/L 2.76mg/L 3.85mg/L 6.41mg/L