Phytoremediation is the use of plant growth to purify pollutants from soil, water, or air. Suppose that a crop of brake ferns can remove 1515 milligrams per square meter of a particular pollutant from the soil in 2020 weeks. After 2020 weeks, the ferns are harvested and a new crop is planted. If cc represents the number of crops of brake ferns needed to phytoremediate soil contaminated with 170mg per square meter of the pollutant down to healthy levels of 5mg per square meter, which equation best models the situation?
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One crop of brake ferns can reduce the pollutant concentration by 15 mg/m^2, so the equation must contain the number 15 multiplied by c, the number of crops of brake ferns.
Since the brake ferns reduce concentration, the sign must be -15c.
The only choice that fits this is Choice 170 - 15c = 5
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