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yellowdoraemon

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belowing is the question:

A river receives a discharge of 10 MGD at a concentrattion of 150 mg/l. The river flow upstream is 20 MGD at zero concentration. For 15 mi downstream, the velocity is 10 mpd. A region of slow moving water is then encountered for next 20 mi where the velocity drops to 2 mpd. If the decay rate of the substance is 0.2/day, what is the concentration at the point 35 mi downstream from the outfall?