Question

An environmental firm is removing pollution from a river bed. They find that the pollution is decreasing at a rate of 5% per month, as measured by parts per million. The pollution starts off at a level of 360 parts per million (ppm).

(A) What is the pollution level one month after they begin clean-up.
(B) What is the level two months after?

Answer 1

(A) 342 ppm

(B) 325 ppm

Explanation:

We assume that 5% of the current level is being removed in the month.

(A)

After 1 month, the pollution level is multiplied by 1-5% = 0.95:

0.95 × 360 ppm = 342 ppm

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(B)

After another month, the pollution level is again multiplied by 0.95:

0.95 × 342 ppm ≈ 325 ppm