We now return to the problem of water pollution of a large lake introduced at the beginning of this section. Suppose a large lake that was formed by damming a river holds initially 100 million gallons of water. Because a nearby agricultural field was sprayed with a pesticide, the water has become contaminated. The concentration of the pesticide has been measured and is equal to 35 ppm (parts per million), or 35 × 10^–6. The river continues to flow into the lake at a rate of 300 gal/min. The river is only slightly contaminated with pesticide and has a concentration of 5 ppm. The flow of water over the dam can be controlled and is set at 400 gal/min. Assume that no additional spraying causes the lake to become even more contaminated. How long will it be before the water reaches an acceptable level of concentration equal to 15 ppm?