Q21 - The trees always blossom

[ManhattanPrepLSAT1]

To move quickly on this question, you don't need to map out all of these answer choices, though you should have the skills to be able to accurately. But you should notice that the sufficient condition is the same for both the conditional premises. Just check for that to match in the answer choices. Hopefully, you know the language cues that imply sufficiency and necessity!

So on answer choice (A) you see that the "if" introduces "garlic in the pantry" for both conditional premises. Check. None of the other answer choices have this same pattern of reasoning.


Stimulus:

AR ---> TBM
AR ---> RF
~RF
========
~TBM

(Notation Key: AR = April rain, TBM = trees blossom in May, RF = reservoirs)

The problem with the reasoning is that first two statements do not allow one to infer TBM ---> RF, though the conclusion would require us to combine them in such a way.

(A) is correct. It has the same flawed structure.

GP ---> GF
GP ---> PBS
~PBS
---------------
~GF

(B) is incorrect. This is a valid argument!

HB ---> OT
OT ---> LI
HB
---------------
LI

(C) is incorrect. This is a valid argument!

200 ---> S
WT ---> 200
~S
--------------
~WT

(D) is incorrect. This is not a valid argument but notice the first two conditional premises. We want them to both begin with the same sufficient condition. These do not.

MO ---> ~EF
PD ---> EF
~PD
-------------
MO

(E) is incorrect. This is a valid argument!

KH ---> PC
PC ---> RO
~RO
-------------
~KH


#officialexplanation

[steven.kantowitz]

I arrived at answer A, but I am unsure why C is incorrect.

[ManhattanPrepLSAT1]

Mind if I ask what your chain of reasoning was? How did you arrive at (A)? What about (A) matched the flaw? In fact... what is the flaw in the stimulus that we are trying to match?

I'll walk you through (C), but I think I can help out more if you show me your thought process was.

[cyruswhittaker]

I initially answered this question incorrectly, but on further analysis, I think I have figured it out:

The argument is flawed due to the causal chain it forms through the contrapositive. At the end of the contrapositive chain (sentence 1), the problem is that "if April..." is a sufficient condition, meaning that the trees could blossom due to some other cause.

We cannot rule out a necessary condition because a sufficient condition does not occur, and this is exactly what the argument does.

Choice A expresses this flawed pattern of reasoning by asserting that the nessary condition (garlic fresh) does not occur based merely on the sufficient condition not occuring.


Is this correct? Please let me know if there are any improvements I can make. Thanks.

[ManhattanPrepLSAT1]

Great work!

I see the flaw in the reasoning as committing a reversal of the first premise. And then attempting to rely on contrapositive reasoning to run backwards. But the difference there is basically two sides of the same coin.

AR ---> TBM
AR ---> RF
~RF
========
~TBM

(Notation Key: AR = April rain, TBM = trees blossom in May, RF = reservoirs)

You're right that through the contrapositive we can establish that the were no April rains, and that would negate the sufficient condition of the first premise. So concluding the negation of the necessary condition would be resting on a negation.

Either way, the argument is heavy in conditional relationships and is fun to play with! Good job on your analysis...

[megm7267]

I'm really confused with this one. Why isn't (E) correct? It seems to have the same exact pattern of reasoning as the question stem.

[samuelfbaron]

I got this answer right and I understand why (A) is correct but could someone help me with understanding sufficient/necessary conditions?

I mapped it out like this:

X --> Y
X --> Z
/Z --> /Y

If rainfall exceed 5 (X) then Trees Blossom (Y).
If rainfall exceed 5 (X) then Reservoirs full (Z).

Reservoirs not full, so trees not blossom. (no z, no y).

So Y and Z both share the same sufficient condition? Is that correct? And just because Z (necessary condition) didn't occur, the the author has falsely concluded that Y (necessary condition) didn't occur?

Thanks as usual!