Formal Logic Problems on Logical Reasoning Forum

[mellyy85]

I understand how to diagram all formal logic statements, and can do the formal logic games, but I seem to have a hard time putting them together for logical reasoning questions. Can anyone give me some advice on how to conquer them? I know they are usually in inference, flaw, and assumption questions, but maybe some strategy on how to attack them for each question type?


Thanks,

Matt L.

[EarlCat]

That's a really broad question. Can you post some specific LR problems you're having trouble with and your thinking on the answer choices?

[Kurst]

Read the chapter on formal logic in the LRB.

[rubydandun]

Read the chapter on formal logic in the LRB.

This. And do the drills until you're absolutely 100% confident with them, to the point where when you see a "diagram" question in LR you can mentally rearrange the sentences/elements in the stimulus, if that makes sense.

[shawn197]

I have the same problem.. I ran through the LR bible exercises, however, my problem more specifically has to do with transcribing LR stimuli into formal logic (I'm pretty good at writing down conditionals tho, as is required in LG).

I'll give you guys an example. On the June 2007 LSAT section 2 question 6.

What can I do to practice transcribing these more complex LR stimuli into pure logic? Any tips.

Thanks

[EarlCat]

I'll give you guys an example. On the June 2007 LSAT section 2 question 6.

What can I do to practice transcribing these more complex LR stimuli into pure logic? Any tips.

Thanks

The premise gives you two conditional statements. First, an undergraduate degree is necessary for appointment to the board. Necessary conditions always go on the right side of the conditional:
AB --> UD

The contrapositive flips and negates the original conditional. Logically, it tells us exactly the same thing as the original statement:
~UD --> ~AB

Second, we're told anyone with a felony conviction may not be appointed to the board (i.e., a felony conviction is sufficient to disqualify you from an appointment to the board). Sufficient elements go on the left of the conditional:
FC --> ~AB
AB --> ~FC

The conclusion says that Murray can't be accepted as Executive Administrator because he has a felony. "Because" indicates sufficiency.
FC --> ~EA

Notice there's a term in the conclusion that is not in the premise (EA), and a term in the premise that's not in the conclusion (AB). This is where the disconnect between the premise and conclusion comes from. Stretch the conclusion beyond the premise as I did below and you can see how the pieces fit together.

Premise: FC --> ~AB
Conclusion: FC ----------->~EA
Combined: FC --> ~AB --> ~EA
Assumption: ~AB --> ~EA (Contrapositive: EA --> AB)