Problem 8. A river with 400 ppm of salts (a conservative substance) and a flow of 25 m /s converges with a stream that has a flow of 5.0 m/s carrying and that has 2.000 mg/L of salts. The salts become uniformly mixed in the river at the point they meet up. A city downstream from this withdraws water from this river and mixes it with pure water (no salt) from another source so that the combined water it uses has no more than 500 mg/L of salts. What is the mixture ratio of pure water to river water the city must use?

Fig: 1