A river with 400 ppm of salts (a conservative substance) and an upstream flow of 25 m/s receives an agricultural discharge of 5 m/s carrying 2000 mg/L salts. The salts quickly become uniformly distributed in the river. A municipality just downstream withdraws water (Q) and mixes it with enough pure water (FQ, no salt) from another source to deliver water having no more than 500 ppm salts to its customers. What should be the mixture ratio F of pure water to river water?