LSAT Question Explanation
PT 105, Section 1, Question 12
Parallel StructureArgument structure
Alice won't volunteer.
Alice will volunteer only if Bruce volunteers and most other volunteers promise to pick Bruce to be manager. No one else will promise to pick Bruce to be manager.
Explanation
This is a valid argument. We're given two necessary conditions for Alice volunteering, and we learn that one of them won't happen. Since we don't have one of the things that's needed for Alice to volunteer, we can conclude that Alice won't volunteer.
Since this is a Parallel Structure question we need to understand the stimulus very well so we can find an answer choice that matches. So to recap: we have two necessary conditions, one is present but not the other, so we correctly conclude that we won't have the sufficient.
Here's a diagram of this proper use of the contrapositive:
E: Alice volunteers → Bruce volunteers and most others promise to select Bruce as manager
C: Most others promise to select Bruce as manager → Alice volunteers
Answer choices
This is a flawed argument, so right away it can't be the right answer because the stimulus was valid. This answer choice negates a necessary condition, but we have no idea if Steve will work alone at the bake sale. Maybe because Bill can't go Steve will just stay home.
Also note that the conclusion of the stimulus says Alice is certain to not volunteer, and the conclusion of (A) says it's likely that Steve will work alone. The conclusion in this answer choice is too weak to match the stimulus.
This is flawed, this is an improper negation. Leon going to the picnic is sufficient for Vera to go, but Leon going is not necessary. The conclusion treats Leon going as necessary for Vera to go. Vera may be fine with going without Leon.
E: Leon goes → Vera goes
C: Leon goes → Vera goes
This is valid and matches the stimulus. Sam and Elaine both going is necessary for Jim to go. And while one of them is going, the other is not. So since we don't have what is needed for Jim to go to the party, we can correctly conclude that he won't go.
E: Jim goes → Sam and Elaine going
C: Elaine going → Jim goes
This is flawed. Paula not working guarantees Jane and Elise will work together, but we have no idea if Arthur would also work with them. We only know Arthur works with Jane when Paula does work with Elise.
This argument is flawed, it's a sufficient/necessary swap. Maria helping is necessary for Therese to work in the yard, but the argument concludes that because Maria will help, it's guaranteed that Therese will work in the yard unless it rains. The argument treats Maria's help as basically sufficient, with the only caveat being a case where it rains.
E: Therese works in yard → Maria helps C: Maria helps → Therese works in yard