Hi @femon90, thanks for your post. These drills test your ability to evaluate a given premise and conclusion, and to ascertain the missing premise needed.
So the way I approach these questions is as follows. What is the given premise? X ==> A. What is the given conclusion? not A ==> B.
Is there a common element? Yes, if you take the contrapositive of the first premise, you get not A ==> not X.
So we could construct a transitive thread: not A ==> not X [missing] ==> B
We have to connect X and B somehow.
Answer choice (A) provides the missing link. It says not B ==> X. The contrapositive would be: not X ==> B. That completes the transitive thread we started above.
Hope this helps! Please let us know if you have any additional questions.
femon90July 18, 2018
Hey I don't understand how to do the missing premise drills. What's the concept?
MehranJuly 20, 2018
@femon90 let's try another 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:
