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Course: LSAT > Unit 1
Lesson 5: Analytical Reasoning – Worked examples- Ordering setup | Overview | Rules and deductions
- Ordering setup | Given info–basic 1 | Worked example
- Ordering setup | Given info–basic 2 | Worked example
- Ordering setup | Given info–could be true | Worked example
- Ordering setup | Given info–cannot be true 1 | Worked example
- Ordering setup | Given info–cannot be true 2 | Worked example
- Ordering setup | Given info–must be true | Worked example
- Ordering setup | New info–could be true 1 | Worked example
- Ordering setup | New info–could be true 2 | Worked example
- Ordering setup | New info–could be true 3 | Worked example
- Ordering setup | Completely determines | Worked example
- Ordering setup | New info-must be true | Worked example
- Grouping setup | Overview | Rules and deductions
- Grouping setup | Given info–basic | Worked example
- Grouping setup | Given info–could be true | Worked example
- Grouping setup | Given info–must be false | Worked example
- Grouping setup | Given info–must be true 1 | Worked example
- Grouping setup | Given info–must be true 2 | Worked example
- Grouping setup | New info–could be true 1 | Worked example
- Grouping setup | New info–could be true 2 | Worked example
- Grouping setup | New info–must be true | Worked example
- Grouping setup | "Completely determines" | Worked example
- Mixed setup | Overview | Rules and deductions
- Mixed setup | Given info–basic | Worked example
- Mixed setup | Given info–could be true 1 | Worked example
- Mixed setup | Given info–could be true 2 | Worked example
- Mixed setup | Given info–must be true | Worked example
- Mixed setup | Given info–cannot be true | Worked example
- Mixed setup | New info–could be true | Worked example
- Mixed setup | New info–must be true 1 | Worked example
- Mixed setup | New info–must be true 2 | Worked example
- Mixed setup | Rule substitution | Worked example
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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.
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.