LSAT 109 | Section 4 | Logical Reasoning: Q5

LR Question 5 Explanation

QUESTION TEXT: Astronaut: Any moon, by definition, orbits some planet…

QUESTION TYPE: Sufficient Assumption

CONCLUSION: Every moon in S4 orbits planet Alpha.

REASONING: Every moon orbits a planet in a solar system.

ANALYSIS: This isn’t a very good argument. The moon’s in system S4 could orbit some other planet in that system (planet Beta?).

But if Alpha is the only planet, then the moons must orbit Alpha, because moons have to orbit a planet.

Answer D and E are trying to trick you into forgetting the conclusion. It’s not enough to know that Alpha has moons orbiting it. We need to know that no other planets have moons orbiting them.

___________

1. It doesn’t matter how many moons there are. We want to know which planet the moons orbit.
2. The moons could all orbit a planet that isn’t planet Alpha.
3. CORRECT. If this is true, then the moons must orbit Alpha. Every moon must orbit a planet.
4. This tells us that Alpha will definitely be orbited by at least one moon. But it also tells us that any other planets will also be orbited by moons.
5. This doesn’t tell us that there are no moons orbiting any other planets.

Recap: The question begins with “Astronaut: Any moon, by definition, orbits some planet”. It is a Sufficient Assumption question. Learn how to master LSAT Sufficient questions on the LSAT Logical Reasoning question types page.