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Course: LSAT (DEPRECATED) > Unit 1

Lesson 5: Analytical Reasoning – Worked examples

Grouping setup | New info–must be true | Worked example

Watch a demonstration of one way to approach a question on the LSAT analytical reasoning section that asks you to find something that must be true, given new information. Created by Annie Hollister.

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Video transcript

- If one of the costumes has green, orange, and white in it, then which one of the following must be true? This questions asks what has to be true give the new information that one of the costumes is green, orange, and white. Let's first add this information to what we already know. We know that indigo and yellow are a pair and if one of the costumes is green, orange, and white then at least one of the other costumes must include indigo and yellow, which we've written here. We also know that red can't be paired with indigo or green and because indigo and yellow are paired, red can't be with yellow, so the only colors red could be with are white and orange, which means that one of the costumes has to be red, white, and orange. This means that if one of the costumes is green, orange, and white, it's this third one. So this gives us a lot of information. In fact, only one of the colors of one of the costumes is not accounted for. So let's look at the choices. Which one of the following must be true? A. All three of the costumes have white in them. Well, we know that two of them do. But we don't know for sure that this third one does so it doesn't look like that has to be true. B. Exactly two of the costumes have green in them. Again, this costume could have green in it but we don't know that for sure so let's leave that off. C. Exactly one of the costumes has indigo in it. Okay, let's look at this. We know that this costume with red in it can't have indigo in it and this costumes that's green, orange, and white also can't have indigo in it so this looks like the answer: Exactly one of the costumes has indigo in it. Let's just double-check the last two choices to make sure they don't have to be true also. D. Exactly one of the costumes has orange in it. Well, we know for a fact that actually two of the costumes have orange in them: the costume that's red, white, and orange, and the costume that's green, orange, and white. So that's definitely not true. And E. At least one of the costumes has both indigo and green. Again, this could either be true or not. This costume with indigo and yellow in it, the third color could be green, orange, or white. We don't know which one so it's not the case that this must be true. So the answer is C. Exactly one of the costumes has indigo in it.