PrepTest 152 - Section 2 - Question 19 | Explanation & Discussion

Support Some species of tarantula make good pets. ████████ ██ ████████ ████ ██████ █████ █████ █ ████ ████ ██████████ ███ ███ █████████ ███████ ████ ██████ ██████

Objective: Parallel Questions

Parallel questions have a highly regimented theory and approach – even if your core logical intuitions are very strong, following a routine process specifically built around the LSAT’s unique patterns will dramatically reduce the time and mental energy required to identify the correct answer. So review these lessons. They’re important.

In short, though, our approach will be to develop an abstract model of the stimulus’ argument, preserving the structure but not the subject matter, then take a shallow dip into the answer choices looking for structural mismatches. Usually that suffices to identify the correct answer, but sometimes we’ll need a deep dive to distinguish between the (usually just two) answer choices that remain after our shallow dip

Use Formal Logic

If thinking about this question in English is easier for you than using formal logic, you need more practice gaining fluency in formal logic. Think of English and formal logic as two closely-related tools, like a hand screwdriver and a power screwdriver. While it’s true that any job you can complete with one you could also complete with the other, they each have niche uses in which they excel. If you find yourself tackling a line of 100 wood screws with a hand screwdriver, you need to get better with the power screwdriver.

Argument Structure – Some Before All

Before diving into the full explanation, I’ll just note that a “thinking in English” approach to this question narrows us down to (A) and (C) without requiring too much advanced knowledge. Perhaps the most realistic approach from a high scorer would be to recognize the “some before all” structure, eliminate answers (B), (D), and (E) on a shallow dip, then loop back to diagram the stimulus upon realizing how closely (A) and (C) resemble it.

Anywho, here’s a walkthrough of all the skills you need to reach the correct argument structure. Starting with the first sentence, which is just a straightforward some claim:

Premise 1: Tarantula ←some→ Good Pet

Interpreting the second sentence correctly means remembering that “no” negates the necessary condition:

Premise 2: Poison Fangs → /Good Pet

Diagramming the conclusion correctly is also a bit tricky – you need to remember that “not all” translates to “some aren’t”:

Conclusion: Tarantula ←some→ /Poison Fangs

So we’ve got one “some” premise, one “all” premise, and a “some” conclusion. We’re also in a normal Parallel question, not a Parallel Flaw question. That’s enough to suspect we’re probably working with a some before all argument structure: if we took the time to figure it all out, the correct diagram would probably match that template, resulting in a valid argument. As mentioned earlier, that’s enough information for a shallow dip. But choosing between (A) and (C) requires a deeper dive, so on with the diagramming we go!

First, here’s the abstract “some before all” structure:

Premise 1: A ←some→ B
Premise 2: B → C
________
Conclusion: A ←some→ C

And here it is again with the premises chained up:

Premise: A ←some→ B → C
________
Conclusion: A ←some→ C

Now here are the three claims from the stimulus:

Premise 1: Tarantula ←some→ Good Pet
Premise 2: Poison Fangs → /Good Pet
________
Conclusion: Tarantula ←some→ /Poison Fangs

Cleanly linking the “some” claim to the “all” claim involves taking the contrapositive of Premise 2:

Premise 2: Poison Fangs → /Good Pet
Premise 2 (CP): Good Pet → /Poison Fangs

Here are the three claims again, with the resulting conditional chain:

Premise 1: Tarantula ←some→ Good Pet
Premise 2 (CP): Good Pet → /Poison Fangs
________
Conclusion: Tarantula ←some→ /Poison Fangs

And here it is again with the premises chained up:

Premise: Tarantula ←some→ Good Pet → /Poison Fangs
________
Conclusion: Tarantula ←some→ /Poison Fangs

So the stimulus does indeed follow the “some before all” structure. That’s the template (A) and (C) are trying to match.

Analysis by MichaelWright

19.

Which one of the following █████████ ██ ████ ███████ ██ ███ ███████ ██ █████████ ██ ███ ████████ ██████

None of the ██████ ███████ ██ ██████ ███ █ ███████ ██████ ███ ████ ██ ███ █████ ██ ████ ██████████ ████ █ ███████ ██████ ██████████ ██ █████ ████ ██ ███ █████ ██ ████ ██████████ ████ ███ ███████ ██ ███████

Some of the █████ ██ ████ ██████████ ████ █ ███████ ██████ ████████ ████ ██ ███ ██████ ███████ ██ ██████ ████ ███ ████ █ ███████ ██████ ██████████ ██ █████ ████ ██ ███ ██████ ███████ ██ ██████ ██ ███ ██ ████ ███████████

Some of the ██████ ███████ ██ ██████ ███ █ ███████ ██████ ████████ ████ ██████████ ████████ ██ ██████ ███████ ██ ███████ ██████████ ███ ███ ██ ███ ██████ ██ ████ ██████████ ███ █ ███████ ██████

tldr: (C) is wrong because the conclusion should have said "Therefore, not all of the poetry with regular meter is in this collection."

(C) should pass your shallow dip, and it’s pretty hard to identify the mismatch unless you’re using formal logic. Casually matching (C)'s concepts up to the stimulus one-for-one, we’ve got a “some” claim in the first sentence, an “all” claim in the second sentence, and a “some” conclusion that combines the first two sentences. Pret-ty good stuff.

Now let’s diagram. The first sentence is Premise 1, and it’s pretty straightforward:

Premise 1: Strawn ←some→ Regular Meter

The second sentence is a bit more complex, but when you work it out it still matches the stimulus. Once again, you gotta diagram that “no” correctly, negating the necessary condition:

Premise 2: Collection → /Strawn

Now I’m gonna show you how to draw a valid conclusion from these two premises. It involves reversing the “some” claim (which is allowed), and taking the contrapositive of Premise 2 like we did in the stimulus:

Premise 1: Strawn ←some→ Regular Meter
Premise 1 (reversed): Regular Meter ←some→ Strawn

Premise 2: Collection → /Strawn
Premise 2 (CP): Strawn → /Collection

Here’s the full valid argument, putting some before all:

Premise 1 (reversed): Regular Meter ←some→ Strawn
Premise 2 (CP): Strawn → /Collection
________
Valid Conclusion: Regular Meter ←some→ /Collection

So. The above diagram shows that you can combine Premises 1 and 2 to validly conclude there is some poetry with a regular meter that is not contained in this collection.

But (C)’s conclusion doesn’t say that. It says something slightly different:

(C)’s Conclusion: Collection ←some→ /Regular Meter
Valid Conclusion: Regular Meter ←some→ /Collection

(C)'s Conclusion: Not all of the poetry in this collection has a regular meter.
Valid Conclusion: Not all of the poetry with a regular meter is contained in this collection.

Structurally, (C)’s conclusion is kinda like the contrapositive of the valid one – it’s flipped and reversed. But you can’t validly take the contrapositive of “some” (or “most”) statements – the two claims are nonequivalent.

Some of the █████ ██ ████ ██████████ ████ ███████ ██ ███████ ████████ ████ ██ ███ █████ ██ ████ ██████████ ███ █ ███████ ██████ ██████████ ████ ████████ ███████████ ██████ ███ █ ███████ ██████

No poems with █ ███████ █████ ███ █████ ██ ████ ██████████ ███ ████ ██ ███ ██████ ██ ████ ██████████ ███ ███████ ██ ███████ ██████████ ███ ███ ██ ███ ██████ ███████ ██ ██████ ███ █ ███████ ██████

PrepTest 152 - Section 2 - Question 19

Difficulty

Analysis

Answer Popularity

Question history

Confirm action