Critical Reasoning – GMAT Question of the Day

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Critical Reasoning – GMAT Question of the Day

This setup is perfect for proof in the strongest terms: the conclusion that is "most properly drawn" will be one that must be drawn — in other words, the answer choice that must be true. We can head straight to the answer choices to establish that proof.

We can look for logical confirmation of the correct answer: first, must (A) be true? No; we are given only percentages, not numbers, and we have no way of inferring numbers. We could have a case in which Paisi's population is very, very small. So (A) is out. Must choice (B) be true? No; these two countries are mentioned in different facts that remain unconnected. It could be that Paisi is very, very small and Kappa is very, very big. Skipping (C) for a moment, we can quickly knock out (D) and (E) also by analysis by cases: we can imagine different cases for countries that haven't been mentioned, or years that haven't been mentioned, and that data could diverge wildly or not at all and still leave the above true.

We're left with (C). Must (C) be true? In the prompt, we have that 20% more people smoke than 20 years ago. Also, the first sentence says that the rate is higher than ever before. Combining them, say the population 20 years ago was 100, and x% of them smoked, which is 100x people. Today, the number of people that smoke is 1.2(100x). But the rate of smoking now is x or greater ("higher than ever before"), meaning that 1.2(100x) divided by the current population is greater than or equal to x:

Multiplying both sides by P and dividing both sides by x, we have:

Indeed, today's population P can be no greater than the population of 20 years ago, which we had picked to be 100 but could have left as a variable. Here we used a technique that is common in GMAT Problem Solving: when working with percentages, try assuming a total value of 100 to make your line of reasoning more concrete.

The correct answer is (C).