Water is flowing through a sewer at a velocity of $0\mathrm{.}1$ m$/$s$.$ The water entering the sewer contains $300$ mg$/$L organic matter If the sewer is $10$ km long and the organic matter degrades according to first order kinetics with a rate constant of $\mathrm{.}1$ d$-1,$ calculate the concentration of organic matter in the water at the end of the sewer. Assume that no mixing occurs in the sewer water.