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Can someone please explain me how the given answer is true

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Can someone please explain me how the given answer is true

by [email protected] » Mon Apr 25, 2011 6:55 pm
If A, then B.
If B, then C.
If C, then D.
If all of the statements above are true, which of the following must also be true?
(A) If D, then A.
(B) If not B, then not C.
(C) If not D, then not A.
(D) If D, then E.
(E) If not A, then not D.

Given answer is C,but I don't understand how it is C. I would rather go with E.
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by vineeshp » Mon Apr 25, 2011 8:23 pm
I dont think GMAT ever asks such questions. it does not clearly say what the circumstances are. So such questions should be avoided.

From your question, we can infer that If A then D.
But it only means that if A happens, then D can happen.
It does not in any place suggest that A is the only way of making D happen.
(If you win, I will give you a treat. This does not mean that if you dont win, there is no chance of me giving you treat. I can give you another treat, say, if i score 760 on the GMAT.)

So E is wrong.

But now with C. It says if not D then not A.
Which means D did not happen. We know that D happens if C, C if B and B is A. So if not D, then A dint happen.
Vineesh,
Just telling you what I know and think. I am not the expert. :)
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by rohu27 » Mon Apr 25, 2011 8:32 pm
IMO questions wth heavy formal logic are not expected on GMAT(an advice frm an expert too few days back in this forum).

but i still tried this Q :D
A)- Q only says if C occurs D occurs but C is not the only way, there may well be others too
B)-if not B then not A is correct, cant say abt C
C) - if not D->not C->notB->notA
D)no mention of E
E)same logic as A

HTH
Cheers,
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by [email protected] » Tue Apr 26, 2011 4:44 am
Thank you both for your response.
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by atulmangal » Tue Apr 26, 2011 7:04 am
Hi,

This is based on a simple concept: the concept is

So if X -> Y then the following conclusions are wrong:

Y -> X
X (NOT) -> Y (NOT)

Only the following conclusion is right:

Y (not) -> X (not)

In your question,

we get, A -> B -> C -> D => A -> D

Hence from u can state that D(NOT) -> A(NOT)
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