September 2009 LSAT Section 4 Question 12

# Which one of the following pairs of employees is such that at least one member of the pair volunteers?

Reply

Shunhe on April 17, 2020

Hi @Maria-Marin,Thanks for the question! Let me answer your questions one by one.

1. We know it’s an in-out game because we know that there are two groups of people: volunteers, and not volunteers. The parents can either be in one group or the other, and there are rules about the parents that say that when one is “in,” another is “out,” and things like this. All of these indicate that it’s a in/out game.

2. So the rule “if not X, then Y” is always going to be an “either or” rule, and not a “not both” rule. Because take a look at what it’s saying. If X is not picked, then Y is picked. Now take a look at the contrapositive, “if not Y, then X.” In other words, if Y is not picked, then X is picked. This is why it’s an “either/or” rule, because one of them is going to be picked. But here’s the thing: let’s say Y is picked. Well, the original condition and its contrapositive only tell us about situations in which one isn’t picked, which means that if Y is picked, it’s consistent with X being picked or not being picked. And vice versa, if X is picked, Y could be picked or not picked. The only situation that can’t happen is that both aren’t picked, but it’s certainly possible that both can’t be picked.

3. (D) is wrong in this question because we know that neither R nor S have to volunteer. Let’s say neither of them volunteer; then we can have V and L volunteer based on rules 3 and 4. And then the other volunteers can be filled in in a number of ways. Indeed, one possible group would just be V and L, with everyone else out, as that doesn’t violate any rules.

Hope this helps. Feel free to ask any further questions that you might have.