draw scematic aetch and list all assumptions$(16$ pt$)$ A river with $400$ ppm of salts $($a conservative substance$)$ and an upstream flow of $25\mathrm{.}0\frac{m^3}{s}$ receives an agricultural discharge of $5\mathrm{.}0\frac{m^3}{s}$ carrying $2,000m\frac{g}{L}$ of salts. The salts quickly become uniformly distributed in the river. A$.$municipality just downstream withdraws water and mixes it with enough pure water $($no salt$)$ from another source to deliver water having no more than $500$ ppm salts to its c$$stomers$.$ What should be the mixture ratio $"F"$ of pure water to river water?