Phytoremediation is the use of plant growth to purify pollutants from soil, water, or air. Suppose that a crop of brake ferns can remove 15 milligrams (\text{mg})(mg)left parenthesis, start text, m, g, end text, right parenthesis per square meter of a particular pollutant from the soil in 20 weeks. After 20 weeks, the ferns are harvested and a new crop is planted. If ccc represents the number of crops of brake ferns needed to phytoremediate soil contaminated with 170\,\text{mg}170mg170, start text, m, g, end text per square meter of the pollutant down to healthy levels of 5\,\text{mg}5mg5, start text, m, g, end text per square meter, which equation best models the situation?