Q19 - My father likes turnips, but

[GRAMMOHAN.BUSINESS]

Why not answer choice D (Erica enjoys studying....) is correct?

[bbirdwell]

Hi R,

This is a "match-the-flaw" question, so your priority is to understand the author's argument, and then find the answer choice that most nearly matches the same (flawed) reasoning.

Answer choice (D) is close, but ultimately off the mark.

In a sense, the original argument says something like this:
X (like turnips), but not Y (like potatoes). Therefore it is NOT true that Y, X.

Thought of the same way, choice (D) would say:
X (like physics), but not Y (like math). Therefore it is NOT true that X, Y.

It seems fine until the very last part, and that's where answer choice (D) makes a subtle mistake. Take a look at the correct answer, (B), and you'll see that it does in fact contain the same exact reasoning as the original argument, though the content is not as similar as the content of choice (D), which is what makes (D) an appealing trap answer choice.

In choice (B), we have:
X (75+ pages), not Y (a novel). Therefore it is NOT true that Y, X.

[ebrickm2]

I get this, but I'm just trying to work through why A is wrong, it's logic is horrible, that much is clear.

conclusion about not part of the group, conclusion about the group.

I guess this is wrong because it is only concerned with one element and doesn't involve two parts. I just hate when choices are so irrelevant and bad, it infuriates me and throws me off the trail that it would be even presented.

[lovelessim]

When I did this problem I discarded "A" because of the word "some". For me, this contrasted too much with the "whoever" in the conclusion "So it is not true that whoever..."

You can probably also discard this answer because of its logic, but this is the method I used.

[bigtree65]

Agreed love that's how I got B instead of a also.
Important question though, whoever is the same as anyone and everyone on lsats right? Is it the same as saying all? Because that's the assumption I made which led me to the correct answer.

[ManhattanPrepLSAT1]

Nice discussion guys!

Agreed love that's how I got B instead of a also.

+1

Important question though, whoever is the same as anyone and everyone on lsats right? Is it the same as saying all? Because that's the assumption I made which led me to the correct answer.

That's correct. Anyone = Everyone = Whoever

(They simply introduce the sufficient condition)

[u2manish]

Dear Brian,

I was wondering if you could help us by diagramming this question. I cant seem to understand how to diagram it: father likes turnips but not potatoes .

Would i be right in doing this:

F----> T and not P

Please also help with the conclusion.

Many thanks,

best M

[mlbrandow]

u2manish,

I looked at the question this way:

Some A are not B.
---> Some B are not A.

This is, at its core, just a mistaken reversal.

Father likes T but not P.
Some people who like T don't like P.
Some who have the quality T do not the quality P.
Some T are not P.

Therefore, it's not true that whoever likes P likes T.
Therefore, it's not true that like P --> like T.
Therefore, some like P do not like T.
---> Some P are not T.

So to recap:

Some T are not P.
--> Some P are not T.

(note that "liking T" and "liking P" have been broadened/simplified to simply T and P. )

But of course we know that it's possible that one is self-contained in the other.

For example, it's true that some quadrilaterals are not squares. Does it follow that some squares are not quadrilaterals? No way!


Now, let's analyze (D) and (B) again:

(D) Some P are not M. Therefore 'All P are M' is false. This is completely valid.

(B) Some >75 are not N. Therefore 'All N are >75' is false. (Therefore Some N are not >75.) This commits exactly the error as in the stimulus--an unjustified reversal of conditions!


Hope this helps!

[nmop_apisdn]

u2manish,

I looked at the question this way:

Some A are not B.
---> Some B are not A.

This is, at its core, just a mistaken reversal.

Father likes T but not P.
Some people who like T don't like P.
Some who have the quality T do not the quality P.
Some T are not P.

Therefore, it's not true that whoever likes P likes T.
Therefore, it's not true that like P --> like T.
Therefore, some like P do not like T.
---> Some P are not T.

So to recap:

Some T are not P.
--> Some P are not T.

(note that "liking T" and "liking P" have been broadened/simplified to simply T and P. )

But of course we know that it's possible that one is self-contained in the other.

For example, it's true that some quadrilaterals are not squares. Does it follow that some squares are not quadrilaterals? No way!


Now, let's analyze (D) and (B) again:

(D) Some P are not M. Therefore 'All P are M' is false. This is completely valid.

(B) Some >75 are not N. Therefore 'All N are >75' is false. (Therefore Some N are not >75.) This commits exactly the error as in the stimulus--an unjustified reversal of conditions!


Hope this helps!

Great explanation, ML!

[WaltGrace1983]

(A)

~P & E
Therefore, ~(P & E)

I have a question. From a purely logical standpoint, wouldn't (A) simply mean that you cannot have P (paperback) and E (expensive) at the same time? That is, it can be a P, but then it definitely is ~E; it can be an E, but then it is definitely ~P. It can be ~P and ~E simultaneously too.

But couldn't "some" also just be a negated conditional, just like what we see in the stimulus? If I say "some A are B," doesn't that also mean that ~(A → ~B)? If so, has this ever been used as a trick? I just want to know, if I see conditional language in the parallel flaw's conclusion, should I get rid of every answer choice that isn't conditional?

(A) would be correct if it said "So it is not true that all paperbacks are also expensive?"

[ohthatpatrick]

Given this conditional:
All girls like Justin Bieber

If I negate the conditional, I'm saying that it's possible to have the SUFF but NOT have the NEC.

It's possible to be a girl who does NOT like JB.

So negating a conditional could equivalently be expressed as:
"it is not true that whoever is a girl likes Justin Bieber"
or
"some girls do not like Justin Bieber"

I can't quite remember them having used that trick, but a weird part of me thinks they have on some modern test. You definitely see some "not all" vs. "some aren't" swaps on Match questions.

(A)'s conclusion IS a conditional. The original argument's conclusion shot down a conditional.

Conditional statements can be thought of as ALL statements. When you say "it is not true that CONDITIONAL" you are negating ALL and getting SOME aren't.

When you negate a SOME statement you get NONE (a conditional). "It is not true that SOME X are Y" is equivalent to saying "No X are Y".

(A)'s conclusion says
Paperback -> ~Expensive
Expensive -> ~Paperback

If I were trying to fix (A), I would say
"This book is not a paperback, but it is expensive. So it is not true that anything that is a paperback is expensive".

(So, yes, you nailed that)

[tatsuaki.tomiyama]

Thanks for the hypothetical alternate A choice at the bottom of the threat. This might save my life.

I initially picked A with confidence because I thought the way it reversed "~ paperback + expensive" to its conclusion was the same as the stim. And right away I hit B and couldn't see the difference between A and B, because it seemed that B has the same reverse or twist like A does.

Having seen your correction of A like above, now, is it correct that the fundamental difference between A and B is that B is more consistent with the stim cuz it got the "all" thing that corresponds to "whoever" in the stim? Or is there any other important logical difference I have to be able to catch?

Thanks so much for all the above comments. Really helpful.

Tommy