Question on Conditional Reasoning

bdfh86
Posts: 1
Joined: Wed Jun 06, 2012 8:42 pm

Question on Conditional Reasoning

Postby bdfh86 » Wed Jun 06, 2012 8:54 pm

I'm trying to map the conditional reasoning in a parallel reasoning question from PT 23 Section 2 Question 25.

Anyway I'm unsure how to write this out:
X->not Y
Y->Z

Therefore does
X->not Z
or does not Z->X

Also I am unsure how to negate conditional reasoning specifically from PT 21 Section 3 Question 4.
I'm trying to negate the answer choices as it is an assumption question. Would a negation of C look like "interests of local constituencies are well served not only by elected officials who do not cater to the interests..." or would it be interests of local constituencies are well served only by elected officials who cater to the interests..."

I've seen the manhattan forums response to this question but I did not completely understand it so any response would be incredibly helpful. Here's the link to that: http://www.manhattanlsat.com/forums/q4- ... 420f79fe40

TERS
Posts: 161
Joined: Thu May 24, 2012 11:29 pm

Re: Question on Conditional Reasoning

Postby TERS » Wed Jun 06, 2012 10:49 pm

I’m no expert, but I’ll take a crack at this.

First, the negation of a conditional statement is a conjunction, not a conditional. For example, the negation of (A --> B) is technically (A ^ ~B). The negation is not, in strict logical terms, A --> ~B. To answer your question, maybe precede the answer choice with “it is not the case that.”

As far as I can tell, the reasoning in Question 25 is valid:

CPES --> ~EAR
ND<5% --> EAR

Thus, CPES --> ~ND<5%

CPES = centrally-planned economic system
EAR = efficiently allocate resources
ND<5% = national debt of less than 5%

The author could reach his conclusion by adding the contrapositive of the second premise to the first, producing a chain wherein CPES --> ~ND<5%.

The correct answer choice is B because it does the same thing, logically (note the first part of the answer choice is the conclusion):

RD --> ~LCA
MAPP --> LCA

Thus, RD --> ~MAPP

RD = rural districts
LCA = large concentration of automobiles
MAPP = major air pollution problems

Again, the conclusion is reached by adding the contrapositive of the second premise to the first, producing a chain wherein RD --> ~MAPP




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