@JanikaBest the key for these questions is the conclusion, so that is where you should focus.
Our conclusion here is: not X ==> A
Right away you should notice that X is not present in P1 or P2, so clearly the missing premise is going to have X.
Now let's look at what we can properly conclude with P1 and P2.
We can combine P1 and P2 using the transitive property as follows:
C ==> D ==> A
This allows us to conclude: C ==> A
Our overall conclusion also includes "==> A" so now we just need to link "not X" with "C" to make this conclusion follow logically.
If we know C is sufficient for A, then not X being sufficient for C, would allow us to properly arrive at A.
So the missing premise here is: not X ==> C
This entire transitive property is:
not X ==> C ==> D ==> A
Hope this helps!
For a more in-depth discussion of these concepts, please watch our video lessons on Sufficient & Necessary conditions and Strengthen questions (because these drills are basically Strengthen with Sufficient Premise questions).
abrielrwmsNovember 29, 2018
The answer was 'not X - > C'. I chose 'C - > not X'. I don't understand the difference.
RaviDecember 20, 2018
Hey there,
"not X - >C" and "C - >not X" mean completely different things, and here's why.
Not X - >C means "if not X, then C." In this statement, not X is the sufficient condition and C is the necessary condition. So whenever not X happens, C must happen. The contrapositive of this statement is
Not C - ->X (this can also be written as /C - >X). This statement means "if not C, then X." This is because if C doesn't happen, then not not X (which is X) must happen."
C - >not X means something completely different. It means "if C, then not X." Taking the contrapositive of this statement, we have
X - ->not C (or X - >/C). In other words, if we have X, then we can't have C.
Do you see how these statements are different now? It's probably a good idea for you to review the video lessons mentioned above in Mehran's comment.
Let us know if you have more questions!
stephanieidemekoMarch 30, 2019
Still confused
JamesMay 19 at 09:53PM
I need a better explanation on specifically why not C --> X
