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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 | 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.
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.