There is usually a scheme in the possible answers to this type GMAT questions. Find details below.

Critical Reasoning Questions are all about distractions, so it is crucial to understand how distractors work. Distractors are commonly referred to as trap-answer choices, and their elimination is never automatic.

Consider this question:

Two farmers, who never left their farms their whole lives, were looking at their dogs and observed that all their border collies were black and white. The border collies were the only things that the farmers ever saw as a mix of black and white. Farmer Gil luckily but correctly observed that if something is a border collie then it must be black and white. Farmer Geva then remarked that if something is not black and white, then it is not a border collie.

The argument is flawed primarily because Farmer Geva:

A. fails to realize that there are other dog breeds apart from border collies B. fails to realize that being black and white is a necessary but insufficient condition to be called a border collie C. lacks sufficient information on which to base the condition for being a border collie D. only observes one type of phenomenon – border collies and their color E. demonstrates only limited knowledge about the world outside

Fairly easily, we can eliminate answers A, D and E.

Answer A is incorrect because it assumes too much – that Farmer Geva is unaware of other breeds. Even if this were true, would this make the argument stronger? Not really, as the argument only needs to deal with one dog breed.

Alternatively, choice D criticizes the argument for considering only one phenomenon. However, this is not a flaw as the argument only deals with one type.

Finally, answer E can also be easily rejected. It mirrors what the premises already state – that they had never left their farms and knew no other dog types except border collies; therefore, this answer choice does not illustrate the flaw.

The toss-up will be between B and C: both answer choices relate to the scope we defined.

At this point we must pause and compare the two answer choices more carefully:

The two statements seem similar, but there are differences. According to answer B, going on the information of Farmer Gil that all border collies are black and white, Farmer Geva supposes that this is enough for a border collie to be called as such, wrongly assuming that only color delineates what a border collie is, whereas there may be other conditions which make border collies what they are.

Answer C, on the other hand, suggests that Farmer Geva lacks sufficient information to be able to define what a border collie is. Clearly this is not the case – the premises state that Farmer Gil correctly points out that all border collies are black and white and also, as one of the conditions for being a border collie (i.e. being black and white) is a premise, then such factual data is considered correct.

Once we isolate this difference, it becomes easier to judge which answer choice is best: Answer C makes a claim that contradicts the premises, whereas answer B correctly indicates a flaw in Farmer Geva’s reasoning.

The correct answer choice is B.

Take home lesson

Remember that GMAT questions often yield to the scheme of 3 + 1 + 1. Three relatively easy eliminations, a distractor and a correct answer choice. Find the three quickly, and then pause for a microscopic look at the last two.

Read the original article on the blog of The Economist's GMAT Tutor.

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