12 Days of Christmas GMAT Competition - Day 3: In the country of Xant

Manhattan Prep Official Explanation

Step 1: Identify the Question

The words assumption and on which the argument depends in the question stem indicate that this is a Find the Assumption question.

Step 2: Deconstruct the Argument

The author tells you about two countries: Xantharu (X) and Tripania (T). In X, only 10% of children have experienced severe lactose episodes, whereas in T, 70% of children have. (Note the reverse wording for T that makes it easy to read the percentage incorrectly!) The author concludes that the inhabitants of X are less intolerant of lactose than the inhabitants of T—while also working in the fact that lactose is found in dairy products that are commonly consumed by children (for example: cow’s milk).

Step 3: Pause and State the Goal

Look for an assumption that the author must believe to be true for the argument to work.

In this argument, the author is relying on statistics of severe lactose reactions to indicate that there is a lower rate of lactose intolerance in X. However, perhaps inhabitants of X are disincentivized to seek medical attention for cases of severe lactose intolerance, resulting in data that presents an inaccurate story. Alternatively, perhaps X has more effective medications to treat lactose intolerance (and so prevent severe reactions) or medications that are more readily available than medications available in T. Or maybe children in X don’t consume as many lactose-based products as children in T in the first place.

Step 4: Work From Wrong to Right

(A) This is a great trap; it’s the opposite of what the author actually assumes. In drawing a comparison between X and T, the author starts from the baseline assumption that it is not the case that children in X simply don’t consume as much lactose. Take this to an extreme: If 90% of X children have never consumed lactose in the first place, then 100% of the ones who did consume lactose exhibited a serious intolerance! That kills the author’s argument. So the author is assuming that the reason the T kids have more reactions is not because they’re more likely to consume a lactose-containing product in the first place—but this choice says the opposite of that.

(B) CORRECT. This one is negatively worded, making it tougher to understand. Basically, it’s stating that the author assumes that both children in X and children in T are equally likely to consume lactose-based products.

A technique known as Negation can help you understand negatively-worded answer choices on Assumption problems (but don’t use this on other CR problem types).To use Negation, take out the negative word and reread the choice. What happens to the argument? If the answer choice is truly an assumption, then it is necessary to the argument, so negating it will make the argument fall apart:

Children in Xantharu ARE not more likely to consume lactose-free milk alternatives than are children in Tripania.

If children in X are more likely to consume lactose-free milk alternatives than are children in T, then inhabitants of X may not have a greater tolerance to lactose than inhabitants of T. Rather, some of them may simply not be consuming lactose in the first place.

(C) This answer choice provides information about additional age groups in X and T, specifically adults and teens. If adults and teens in T experience higher rates of severe lactose intolerance than adults and teens in X, that certainly strengthens the author’s argument. However, the numbers listed here are very specific—too specific to be necessary to the author’s argument. The author doesn’t need to assume these specific percentages in order for the argument to work.

(D) The argument mentions only percentages, which indicate a relationship of a subgroup (those who experience severe lactose intolerance) to the main group. Regardless of how large the population is, these percentages remain the same. For example, it doesn’t matter whether X has a population of 1,000 children and 100 experience severe lactose intolerance or whether X has a population of 10,000 and 1,000 of them experience severe lactose intolerance—either way, it’s still 10% of the population.

(E) Like answer (C), this choice is too specific. While the author’s argument does seem to assume that inhabitants of X adapted to have greater lactose tolerance, the author never claims a specific time frame during which that adaptation must have occurred.