Don't Cave to Diagramming
Test 20 - Section 4 - Question 15
This is the type of question where, as a rookie LSAT teacher hired over the telephone by one of the big prep companies a decade ago, I would have drawn a diagram. I'd bust out the dry erase marker and do a big fancy diagram of linked conditional statements—making sure to also show the contrapositive of course—because "that's how the LSAT is done." The top 1/3 of my class would be bored, finding the whole exercise far too obvious. The middle 1/3 would take notes feverishly, hoping to get the hang of it. The bottom 1/3 would stare back blankly, believing (probably correctly) that they would never be able to understand these crazy LSAT hieroglyphics.
As a more experienced LSAT teacher, I've dumped this old LSAT diagramming dogma almost entirely. It just doesn't help anyone.
Top students don't need a diagram—they can do it far more quickly and reliably in their heads. Everyone else is prone to all sorts of errors. If you can't follow the argument in your head, you're going to have a hell of a time boiling it down into abstractions.
Just take it one step at a time and you'll be fine. The first sentence says Rhonda will only see the movie if Paul goes to the concert. The "only if" introduces a necessary condition, so if Rhonda goes to the movie it means we know for sure that Paul went to the concert. (Looking at it another way, if Paul doesn't go to the concert then Rhonda can't see the movie.)
The next sentence says Paul won't go to the concert unless Ted goes to the concert. "Unless" means "if not," so we can translate that into "If Ted doesn't go, Paul doesn't go."
And we can link those two statements together in our heads. If Ted doesn't go to the concert, then Paul can't go to the concert. And if Paul doesn't go to the concert, then Rhonda can't see the movie.
As it turns out, that last line is exactly what the conclusion of the argument actually says. So here we have a valid logical argument using two premises that link together. It's the most common pattern of valid reasoning on the LSAT. It's like saying "You can only be in Las Vegas if you're in Nevada, and you can't be in Nevada unless you're in the United States, so if you're not in the United States you're not in Las Vegas." Is that really so hard? It's just common sense, isn't it?
A) This answer is logically invalid. If Janice doesn't get a babysitter, she won't visit Mary. But if Janice does get a babysitter, there's nothing forcing her to visit Mary. A babysitter was necessary for her visit, not sufficient. This can't be the answer because the given argument was logically valid.
B) Perfect. Cathy's not ill, so Peter doesn't have to go to work. Since Peter doesn't have to go to work, Gary won't do his laundry. That's exactly what the conclusion says, so this is a logically valid argument linking two premises together.
C) We can stop reading this one after the first sentence. The premise "Kelly will barbecue fish tonight if it does not rain and the market has fresh trout" has a joint sufficient condition (not rain and fresh trout). There was nothing like that in the given argument or in B, so there's no need to waste more time on this answer.
D) Nah, get out of here with this "one of her brothers" business. There's nothing like that in the given argument, and we already have a perfect match with B.
E) The whole "most" and "some" business gets rid of this one.
The correct answer is B because it's a valid argument linking two conditionals together. A diagram *might* be useful for teaching purposes but I'd never do it on the actual test. And it tends to make students think that the LSAT is harder than it actually is. So why even bother teaching it?