PSAT Math Multiple-Choice Question 64: Answer and Explanation

Question: 64

An oceanographer is trying to determine the concentration profile of dissolved oxygen in the ocean at depths lower than 10 meters. He determines that the dissolved oxygen concentration is 0.0022 g/L at a depth of 15 meters, and it is 0.00125 g/L at a depth of 20 meters. If C(d) is the concentration of dissolved oxygen at d meters, which of the follow equations best describes the profile below 10 meters?

• A.
• B.
• C.
• D.

Correct Answer: D

Explanation:

D

The question asks for an equation that best models a specific situation. The question says that at a depth of 15 meters, the dissolved oxygen concentration is 0.0022. Plug d =15 into the answers to see which one equals the corresponding oxygen concentration of 0.0022. Choice (A) becomes . In decimal form, the expression is . This does not match the target number, so eliminate (A). Choice (B) becomes . In decimal form, this is equal to 0.0006. Eliminate (B). Choice (C) becomes = 0.06. Eliminate (C). Choice (D) becomes . Continue simplifying to get . This is a very close match for the target. The correct answer is (D).