The concentration of treated sewage in a river just downstream of a sewage-treatment plant's effluent pipe is 75 mg/L.

The concentration of treated sewage in a river just downstream of a sewage-treatment plant's effluent pipe is 75 mg/L. If the organic material of sewage origin is destroyed through a first-order reaction with a rate constant equal to 0.05/day, what is its concentration 50 km downstream? The velocity of the river is 15 km/day.