#21 - Most kinds of soil contain clay, and virtually every

[Administrator]

Complete answer explanation.


Parallel Flaw. The correct answer choice is (E).

Most kinds of soil contain clay:

Soil clay

Virtually every kind of soil (most) contains either sand or organic material or both

Soil sand or OM or both

Therefore, there must be some soil that contains both clay and sand (C+S) and some that contain clay and organic material (C+OM)

Soil (C+S)

Soil (C+OM)

We are looking for a parallel flaw here, so we can look for an answer choice with a pattern of two most statements followed by a some statement. Additionally, we're looking for the matching flaw - here the statement says that most soil contains either sand or OM or both - that means all soil could just contain sand. Therefore we cannot say that there must be some soil that contains clay and organic material.

Answer choice (A) - This answer doesn't match because it adds a conditional "if there are pharmacies that sell both, then there must also be..."

Answer choice (B) - this logic works backwards, starting with a statement that "most pharmacies sell cosmetics becasue.." this does not match the pattern in the stimulus.

Answer choice (C) - this answer choice adds an unless statement in the conditional, so it doesn't match

Answer choice (D) - this question has the most / most/ some pattern, but it doesn't have the flaw from the simulus because it doesn't allow for a pharmacy to sell either/or product. The inference drawn is proper logic, so it cannot be the parallel flaw we're looking for

Answer choice (E) - this is the correct answer. Most pharmacies sell either shampoo or toothpase, most pharmacies sell makeup, therefore some pharmacies must sell one of these two combinations. Like the stimulus, it overlooks that all pharmacies could sell only shampoo, and so there might not be a pharmacy that sells cosmetics and toothpaste.

[mokkyukkyu]

Hi,

I chose A...
I thought the structure is Most of X contains A and all of X contain either B or C, or both.
Thus, there are some of A that contain B and C.
Actually I thought this is not a flaw because it's the combination between All and Most statement so we can infer Some...
What makes A wrong and E correct?

Thanks,

[Nikki Siclunov]

Hi mokkyukkyu,

Technically, you are correct. The following combination of statements would indeed produce the inference you indicate:

• A B
  A C
  ===========
  B C

The stimulus, however, is a bit more complex. Let's take a look:

• Soil Clay
  Soil Sand or Organic Material
  ============
  Clay Sand
  Clay Organic Material

The conclusion is not logically valid. Do you see why? If soil always contained both sand and organic material, the conclusion would have been fine. However, apparently soil needs to contain one or the other (or both) of these materials. Since only one of the two necessary conditions needs to be satisfied, it's possible that no soil contains sand (if all soil contained organic material), and it's also possible that no soil contains organic material (if all soil contained sand). Thus, no conclusion can plausibly be drawn on the basis of the information presented.

To attack this Parallel Flaw question, use this insight to identify an answer choice that contains the same error in reasoning.

Hope this helps!

Thanks,

[bli2016]

Hi, I read the response above and seem to understand the problem better now but I just wanted to double check my reasoning for eliminating A. Is it because the last sentence is a conditional statement that never appears in the stimulus (if there are pharmacies that sell both cosmetics and toothpaste, there must also be some that sell both cosmetics and shampoo)? Thanks.

[Ricky_Hutchens]

Hi bli2016,

Exactly. That last sentence in A is a conditional statement that has no parallel in the original stimulus.

[chian9010]

Hi,

I wasn't sure if I understand the flaw correctly.
If I draw the diagram,

Most kind of soil contain clay
Soil clay

Every kind of soil contains either sand or organic material or both

Soil sand
soil organic material
soil both

Combine with the first promise

sand soil clay (some soil contain both clay and sand)
organic material soil clay (some soil contain both clay and organic material)


It sounds logical for me. However, if we assume 80% of the soil has clay. Then, it is also possible that the rest 20% of the soil which doesn't contain clay contains sand. In other words, there is NO MUST some kinds of soil contain both clay and sand.

I don't know why my diagram will reach a different result as my percentage example. Is there anythig wrong with my diagram?

[Rachael Wilkenfeld]

Hi Chian

Saying that virtually every kind of soil contains sand or organic material or both does not mean that any soil contains sand. It could be that it's all organic material. Or it could be that all the soil contains sand, but none contains organic material. You can't break up the statement in the way you attempted.

Hope that helps.
Rachael

[mshaheen14]

Hi Powerscore,

I understand why (E) is the correct anwser, but is the only reason that (D) is incorrect is because it fails to say "or both" at the end of its first premise (Virtually every pharmacy that sells shampoo also sells toothpaste).

I found that all other aspects of answer choice (D) line up very well with the stimulus and therefore ended up picking it. It also matched my paraphrase: Most A, Virtually every time B or C. Therefore there is A&B and A&C.

Thanks!

[Adam Tyson]

The first problem that should catch our attention with answer D, mshaheen14, is that the premise beginning with "virtually every" (the first sentence of the answer) does not match the "virtually every" premise in the stimulus. Using some very abstract language here, in the stimulus it was "virtually every X has Y or Z or both," but answer D's premise is structured as "virtually every X that has Y also has Z." It's about virtually all of the X's that have Y, rather than being about virtually all of the X's! To come back to the details, we needed an answer about virtually all pharmacies, not an answer about virtually all pharmacies that sell shampoo (adding a qualifying characteristic to the category). And then you're right, the premise fails to have any element of "or both" to it, which is the final nail in the coffin for this answer.

[Liz0501]

Hi! I'm confused as to why the conclusion isn't diagramed as

soil clay and sand
soil clay and organic material

Thanks!