Application of Ordinary differential equations

Question

A large lake is to be formed by damming a river which holds initially 100million gallons of water. The water got contaminated with a pesticide of concentration of 35ppm (parts per million). The river continues to flow into the lake at a rate of 300gal/min. The river is only slightly contaminated with pesticide and has a concentration of 5ppm. The flow of water over the dam can be controlled and is set at 400gal/min. How long will it be before the water reaches an acceptable level of concentration equal to 15ppm? Please help me, all I know is that, it is flow rate problem