Ariver has a flow velocity of 3 cm/s, an aeration coefficient of 0.5 d–1, and a temperature of 20°c. after initial mixing with wastewater, the river has a do concentration of 8.0 mg/l, an ultimate bod of 25 mg/l, and a bod decay constant of 0.8 d–1. neglecting the effect of sedimentation on bod removal, determine the distance over which the do is less than or equal to 7 mg/l.