$1\mathrm{.}5$ A river with $400$ ppm of a salt $($a conservative subrtance$)$ and an upstream flow of $25\frac{m^3}{s}$ receives an agricultural discharge of $5\frac{m^3}{s}$ Carrying $2000m\frac{g}{L}$ of salts $($Fig $6$ elow $).$ The sait quickly become unifurmly distributedin the river. A municipality just downshean withoraws water and mixes it with enough pure water $($no salt$)$ from anather source to deliver water having no more than $500$ ppm sates to its custamers. What should be the mixture ratio $F$ of pure water to river water?

$(8)$