@Sarah194747 Let's try a missing premise drill together to see if it helps:
P1: A ==> not B B ==> not A ​ P2: ?
C: A ==> X not X ==> not A
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 "A".
What does the existence of A tell you?
Well from P1, we know that if you have A, then you don't have B ("not B"):
A ==> not B
But is that our conclusion? No!
The conclusion is "X".
So the jump here is from "not B" to "X".
Since we have "not B", we want "not B" to be sufficient for "X" as that will close the transitive chain as follows:
A ==> not B ==> X
Contrapositive of this chain is:
not X ==> B ==> not A
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:
