## help with formal logic question

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walterwhite

Posts: 178
Joined: Thu Jul 18, 2013 8:31 pm

### help with formal logic question

Can somebody help me with question 16 from section 4 of preptest 38? This is a good example of questions I struggle with. Do I necessarily need to diagram this kind of question? The power score bible says for assumption questions you should look for elements missing from the premises that appear in the conclusion, or elements missing from the conclusion that appear in the premises. Does the power score method require diagramming the sufficient and necessary conditions?

neprep

Posts: 1066
Joined: Fri Jul 26, 2013 11:16 pm

### Re: help with formal logic question

Diagraming can certainly help on some sufficient assumption questions like this one. The LRB wouldn't classify this question as an assumption question; it would classify this as a "Justify" question. This is not to say that its general approach to assumption questions cannot be applied here. You are still looking for elements in the conclusion that seem to appear out of thin air:

In this question, the premises lay out two facts about people who do not believe others distrust them (Group A): (1) They are confident in their abilities and, as such, (2) they think of a difficult task as a challenge rather than a threat.
From these premises, the argument concludes (2) about people who tend to trust others (Group B)*, but has provided no information about Group B. But if we were to assume Group B = Group A, however, then the conclusion follows, because then whatever holds true for Group A also does for Group B.

You can also diagram this, I guess:

Premise: Group A --> Confident In Their Own Abilities --> Think of Difficult Task As Challenge
Conclusion: Group B --> Think of Difficult Task As Challenge

Assumption, which is choice (C), says Group B --> Group A, therefore Group B --> Think of Difficult Task As Challenge.

This codified method is essentially saying the same as the thinking-it-through method, except it may take a bit more time. Also, you should check out The LSAT Trainer's chapter on sufficient assumptions. It might help with speed. --LinkRemoved--

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*LRB would classify this as a "rogue" element, I think, if that helps because you're accustomed to its parlance.

walterwhite

Posts: 178
Joined: Thu Jul 18, 2013 8:31 pm

### Re: help with formal logic question

you're right it is not an assumption question! the LRB makes it a little difficult to distinguish between assumption and justify the conclusion questions. also with LR questions like this I have a hard time sorting out what's a necessary condition and what's a sufficient question.

sidenote- i find the terms "sufficient" and "necessary" confusing. i mean i understand that the necessary condition "necessarily" results from the sufficient condition but when you're in the middle of a tough question like this it's hard to keep it all straight

neprep

Posts: 1066
Joined: Fri Jul 26, 2013 11:16 pm

### Re: help with formal logic question

walterwhite wrote:you're right it is not an assumption question! the LRB makes it a little difficult to distinguish between assumption and justify the conclusion questions. also with LR questions like this I have a hard time sorting out what's a necessary condition and what's a sufficient question.

sidenote- i find the terms "sufficient" and "necessary" confusing. i mean i understand that the necessary condition "necessarily" results from the sufficient condition but when you're in the middle of a tough question like this it's hard to keep it all straight

Yep, I think that's one of the few shortcomings of the LRB. Basically, when asking for a necessary assumption, the question stem will make it clear that the answer choice is something on which the argument "depends" or something that it "requires": kinda like life support equipment for a patient on the verge of death. Pull the plug and the lights go out. That was macabre; sorry 'bout that.

When asking for a sufficient assumption, the question stem will indicate that the right answer will be "logically" concluded from the premises or justify the argument with brute force.

This topic has been discussed ad nauseum on this forum though, so a little searching around on this topic and you'll be set.