Question

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(mg) 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 c represents the number of crops of brake ferns needed to phytoremediate soil contaminated with170 mg per square meter of the pollutant down to healthy levels of 5 mg per square meter, which equation best models the situation?
A) 170 - 20c = 5
B) 170 + 20c = 5
C) 170 - 15c = 5
D) 170 + 15c = 5

Answer 1

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 C) 170 - 15c = 5