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Course: LSAT (DEPRECATED) > Unit 1

Lesson 5: Analytical Reasoning – Worked examples

Grouping setup | Given info–must be true 2 | Worked example

Watch a demonstration of one way to approach a "Given info: must be true" question on the analytical reasoning section of the LSAT. Created by Annie Hollister.

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Video transcript

[Instructor] Which one of the following must be true? These questions ask what must be true given the rules of the setup and anything we can infer from that. So before we look at the choices, let's think about what we can figure out from the rules of the setup. We know that any costume that has indigo in it must also have yellow in it. And any costume that has yellow in it must also have indigo in it. So that means that indigo and yellow are a pair, which means that at least one costume has indigo and yellow in it. We also know that if a costume has red in it, then it can have neither indigo nor green in it, which means that red can't be with indigo or green. But since indigo and yellow are paired, it also means that red can't be with yellow, that red can only be paired with orange or white. And since each costume is three colors, that means that one of the costumes is orange, red, and white. That's the only possible color combination that involves red. The rule one tells us that no two costumes can have the same color combination. Since there's only one possible color combination that involves red, that means that only one costume can have red in it. And we know that at least one costume has red in it because the rules tell us that every color appears at least once. So the answer is B, exactly one of the costumes has red in it. We want to double check that the other ones could be false. For instance, exactly one of the costumes has indigo in it. Well, this could be false if the third costume also has indigo and yellow as long as the third color in those costumes doesn't match. So that's fine. Exactly two of the costumes have green in them. Well, that could be false. For instance, if the grouping looks like this. Exactly two of the costumes have white in them. Again, that could be false. For instance, if three of the costumes have white in them. And finally, any costume that has orange in it also has white in it. That could be false, for instance, if the grouping looks like this. So the answer is B. Exactly one of the costumes has red in it.