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Philippa on November 7, 2018

Sufficient and Necessary Premise Drills

Hello, I'm having trouble with the Sufficient and Necessary Premise Drills. I have watched all the videos under the topic and memorized the key terms. I know how to diagram based off of a passage; but with the premise drills I don't know what particular rules to follow and I don't get the sequence.Is there any trick or something I can follow to help me understand? Thanks

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Mehran on November 7, 2018

Let's try a missing premise drill together to see if it helps:

P: B ==> Not X

P: ?

P: Not A ==> Z

C: Not Z ==> Not X

Okay so first these are Strengthen with Sufficient Premise questions in the abstract.

The idea is there is a gap in the argument and we are looking for the answer choice that would make the conclusion follow logically.

Notice the sufficient condition of your conclusion is “not Z” so that is where I will start.

Contrapositive of P3 is Not Z ==> A

So we have A but then we are stuck because A is not a sufficient condition of any of our premises.

So the jump here clearly will relate to A.

We are trying to get to “Not X” because that is your necessary condition of the conclusion.

P1 gives us “Not X” because “B ==> Not X”

So if we can connect A to B we would complete this argument logically as follows:

not Z ==> A ==> B ==> not X

So the missing premise is A ==> B.

Hope that helps!

If you would like to see this concept taken out of the abstract and applied on an actual LSAT questions, please review this question from the June 2007 LSAT: https://testmaxprep.com/lsat/lsat-practice-videos/june-2007-lsat/section-2-question-6-explanation.