#24 - In the Riverview Building, every apartment that has a

[Administrator]

Complete Question Explanation

Parallel Flaw—SN. The correct answer choice is (C)
This author presents several conditional statements regarding the apartments in the Riverview Building. First of all, every apartment that has a balcony also has a fireplace:

• Balcony Fireplace

…further, if an apartment has a balcony, it is not a one-bedroom apartment:

• Balcony one-bedroom

Because the two conditional statements presented in the stimulus have the same sufficient condition, they can be joined in a single diagram as follows:

• Fireplace
  Balcony +
  one-bedroom

The author then invalidly draws a conclusion about the relationship between the two necessary conditions, wrongly claiming that none of the one-bedroom apartments have a fireplace:

• one-bedroom Fireplace

This is a unique flaw: it looks as though the author invalidly linked the two necessary conditions:

• fireplace one-bedroom,

and then drew the contrapositive of the invalid statement:

• one-bedroom Fireplace.

The question that follows asks for the answer choice that reflects an analogous flaw.

Answer choice (A): The premises presented in this choice are as follows:

• There is no cat that lacks fur: cat lack fur
  
  There is no cat that is a fish: cat fish
  
  The conclusion is that every fish has fur: fish lack fur

Although this choice does reflect flawed logic, it does not parallel the flaw from the stimulus (as only one of the two necessary conditions is negated), so it should be eliminated.

Answer choice (B): This choice bases its conclusion on the following two premises:

• Every dog has fur: Dog Fur
  
  No cat is a dog: Cat Dog
  
  Contrapositive: Dog cat

The first premise can be linked with the contapositive above as follows:

• Fur
  Dog +
  cat

The conclusion presented in this choice is that Some cats lack fur:

• Cat fur

The conclusion above is certainly flawed, but this is different from the logical flaw reflected in the stimulus, this cannot be the right answer to this Parallel Flaw question.

Answer choice (C): This is the correct answer choice. The premises presented in this choice are as follows: First, every cat has fur:

• Cat fur

Next, no cats are dogs:

• Cat dog

As with the conditional statements presented in the stimulus, the two statements listed here share the same sufficient condition and can thus be linked diagrammatically as follows:

• Fur
  Cat +
  Dog

To apply the same flawed logic to this statement as the author did in the stimulus, we would link the two necessary conditions (invalidly):

• Fur dog

and draw the contrapositive, concluding, as this choice does, that:

• Dog fur

Since this choice makes the same conditional reasoning error as the one in the stimulus, this is confirmed as the right answer to this Parallel Flaw question.

Answer choice (D): This choice presents the following argument:

• Premise: No cat is a dog: Cat Dog
  
  Contrapositive: Dog Cat
  
  Premise: No dog is a fish: Dog Fish

Linking together the information from these premises, we can say with confidence that a dog is neither a cat nor a fish:

• cat
  Dog +
  fish

But this does not lead to the conclusion that every cat is a fish:

• cat fish

Although this reasoning is flawed as well, it does not parallel the flaw reflected in the stimulus, so it cannot be the correct answer to this parallel flaw question.

Answer choice (E): The premises and conclusion presented in this answer choice are as follows:

• Premise: every dog is a mammal: dog mammal
  
  Premise: no fish is a mammal: fish mammal
  
  Contrapositive: no mammal is a fish: mammal fish

The first premise can be validly linked with the contrapositive directly above as follows:

• dog mammal fish

So, if something is a dog, it is not a fish, and, on the contrapositive side, if something is a fish it is not a dog:

• fish dog

This confirms the conclusion in this answer choice, that no fish is a dog.

Since this choice reflects valid logical reasoning, it cannot possibly parallel the flawed logic in the stimulus.

[EL16]

Hello,

I am having trouble understanding why C is correct over D. I have looked at this problem repeatedly, and find that both C and D work. I have looked at the explanation above, and it still seems like C and D are both correct according to that logic as well. Could someone please scan through my diagramming/logic below and help me identify the problem? Thanks in advance!

Stimulus:
Balcony fireplace
Balcony NOT one bedroom
Conclusion: one bedroom NOT fireplace

C:
Cat fur
Cat NOT dog
Conclusion: dog NOT fur
*C aligns well with the stimulus because the conclusion is (incorrectly) taking the 2 necessary conditions identified in the premises (fur and NOT dog), and linking them together, and negating them both (so fur becomes NOT fur and NOT dog becomes dog)

D:
cat NOT dog
*contrapositive: dog NOT cat
dog NOT fish
conclusion: cat fish
*D aligns well with the stimulus because the conclusion is (incorrectly) taking the 2 necessary conditions identified in the premises (NOT cat and NOT fish), and linking them together, and negating them both (so NOT cat becomes cat and NOT fish becomes fish)

Is D incorrect because of that contrapositive that was required, whereas the stimulus did not require a contrapositive to be taken? Even if thats, the case, it still seems to me that the reasoning is paralleled, so I am confused about when it is appropriate to take a contrapositive versus when it is not.

Thanks,
Elana

[Adam Tyson]

Good analysis, Elana, and good question! It's true that sometimes taking a contrapositive is an acceptable way to find a parallel argument - but it's rare, and if there is another choice that doesn't require it then that other choice is better. Since we are not tasked with picking "correct" answers, but rather with picking the "best" answer, you have to go with C in this case for that reason.

But wait, there's more! Another way to see why C is the better choice is go beyond just the idea that the necessary conditions are simply "linked" to see that the link is a claim that they are mutually exclusive. That is, the argument is saying that one of those things CANNOT be the other thing. If this was a logic game we would diagram that with a double-not-arrow:

One Bedroom Fireplace.

In answer choice D, the relationship is a standard conditional claim, saying that one thing MUST BE the other thing:

Cat Fish.

That, too, is not quite parallel to the stimulus, and since answer C has the right type of double-not-arrow relationship (the two are mutually exclusive) while D has a standard conditional relationship (one of the two must be the other), it's not quite the same.

C is better than D for both of these reasons, Elana, and "better" ("most effectively" in the case of this particular question stem) matters on this test. Go for the best!

[EL16]

Thank you, Adam! Great explanation.

[lsatstudier1]

Hello,

I am wondering what the best approach to a question like this would be. On my first attempt, I solved the question incorrectly and after looking at the post's methodology to solving the first answer choice, it took me about 3 minutes to go through the rest of the answer choices.

I am taking the LSAT in 6 weeks and want to score around a 170: considering this, would my best bet be to skip a question like this? Or how can I improve my speed? Do you have any tricks?

Thank you!

[Rachael Wilkenfeld]

Hi lsatstudier1,

There are a few things you can do to manage speed here. I would certainly diagram the stimulus clearly, to have something to compare the answer choices to. I'd also quickly eliminate answer choice (B) because it talks about "some" whereas our stimulus does not use that quantity. I'd also eliminate answer choice (E) quickly because it's valid reasoning, and we know we are looking for flawed reasoning.

From there, you have three answer choices left to diagram, which should be doable in a reasonable amount of time---likely more than the 1 minute, 25 second average, but not too much more. You'll also be at a 1/3 shot by eliminating the two quick wrong answers, instead of your initial 1/5 odds. By question 24, you'll know if you have the 2ish minutes to spend on this question, or if you need to guess and come back if time allows That might vary for you from test to test.

I would say that for accuracy reasons, this question really does require a fair amount of diagramming. It can be easy to get conditions turned around otherwise. If I saw this on test day, I would almost certainly need to diagram multiple answer choices as well as the stimulus.

Also, remember that a 170 does not require perfection. It requires a very high degree of accuracy. But you can skip or guess on a few questions, and still score in that range.

Hope that helps!
Rachael

[stephee490]

Hello, I am confused about the modifier "None". I assumed None would produce a reversible arrow, and therefore the statement would follow as

1 bedroom apartment w balcony fireplace

Can someone explain when do I avoid using the reversibility rules?

[stephee490]

Hello, I am confused about the modifier "None". I assumed None would produce a reversible arrow, and therefore the statement would follow as

1 bedroom apartment w balcony fireplace

Can someone explain when do I avoid using the reversibility rules?

I went over the formal logic chapter and found how to solve it with the double not arrow actually. I also located the premises and conclusion to matches the conditional statements

1 bedroom apartment w balcony fireplace produce the inference of fireplace ~1 bedroom ( bedroom is negated/crossed out)
while the author concludes that 1 bedroom fireplace, WHICH IS INCORRECT.

Conclusion is in bold and premise in underlined (since is a premise indicator)

So the one that matches the faulty conclusion is C) no dog has fur since every cat has fur and no cat is a dog

conclusion: dog fur
premise: dog cat fur

D was tempting because it had every in the answer choice as it did in the stimulus. However, the conclusion does not match up further more the conditional statements of the premise don't match either.

If I made any mistakes please comment

[Robert Carroll]

stephee490,

Looks like that went well! A Double-Not Arrow is reversible, but that does no good here:

1 bedroom apartment w balcony fireplace

equals

fireplace apartment w balcony 1 bedroom

There's still no inference possible. A Double-Not Arrow leads to an inference with a conditional when it's on the other side of the conditional:

A B C

leads to

A C

I think you got this completely in your second post, but I'm just posting for anyone else!

Robert Carroll