Problem $8$:

A river above a town has a flow of $200\frac{L}{s}$ and BOD of $1m\frac{g}{L}.$ The river is receiving water for $25\frac{L}{s}$ of treated sewage $($BOD $=25m\frac{g}{L})$ from the town and $25\frac{L}{s}$ of untreated effluent $($ BOD$=175m\frac{g}{L})$ from a pulp and paper mill just downstream from the town. If the BOD of the river below the mill outfall must not exceed $5m\frac{g}{L},$ what percent removal of organic matter $($BOD$5)$ is required by the mill to meet this requirement?