Leachate is a solution, frequently highly contaminated, that develops

Hi,

an inference is an implied conclusion. But on a GMAT inference question, if you see an answer choice that must be true based on the stimulus, you should always automaticlaly select it. There are two reasons for this: a) what you think is a simple paraphrase may actually have something new in it, and can thus be regarded as a proper inference; and b) some questions simply ask you to treat the passage as true, and select a choice that "must be true"--in these cases, the correct answer may even be a restatement of one of the statements in the passage.

In a GMAT inference question, a choice that must be true based on one or more statements in the passage is ALWAYS automatically correct.

Thus, the the four wrong answers are things that could be false.

Let's apply this approach to this question.

In the passage we learn that leachate gets into the environment if and only if the landfill’s capacity to hold liquids is exceeded. This means two things. First, whenever the landfill's capacity is exceeded, some leachate will definitely escape into the environment. Second, if the landfill's capacity is NOT exceeded, then there is nothing to worry about. (Formally, the landfill's capacity being exceeded is both a sufficient and necessary condition for leachate escaping into the environment).

Let's consider choices D and E:

(D) Some landfill leachate is send to sewage treatment plants that are incapable of handling it.

Well, I can see why this is tempting. After all, the last sentence told us that leachate (well, most of it anyways) is sent to sewage treatment plants, and that some sewage treatment plants can't handle leachate. But did we learn whether any leachate goes to the plants that can't hanlde it? Nope. Fine, there may be some plants that can't handle leachate, but it could easily be the case that no leachate is actually sent to these plants--that leachate is only sent to plants that CAN handle leachate.

Thus, choice D could be false, and we can eliminate it.

(E) If leachate does not escape from a landfill into the environment, then the landfill’s capacity to hold liquids has not been exceeded.

Well, we learned that if the landfill's capacity were exceeded that leachate will for sure escape into the environment. So, if no leachate escaped, then capacity was not exceeded (because if capacity were exceeded, leachate would have escaped!). (Many of you will recognize this concept as the "contrapositive").

Thus, choice E must be true (based on the passage). Thus, choice E must be correct.

TAKEWAY: in GMAT inference questions stay very CLOSE to the passage. If you see a choice that must be true, it is undoubtedly the correct answer. If you see a choice that could be false, it is undoubtedly an incorrect answer.

SOMETHING YOU SHOULD KNOW ABOUT THIS QUESTION: this is an LSAT or else an LSAT-like question. It tests formal logic in a way that GMAT CR never would.