CR need ex based deductio

[vanD]

Source: 1000CRs of scoretop.com -- problem#7

7. 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.

[Saurabh Malpani]

The answer should be B in my opinion.


Reasoning here is:
(A) If D, then A.----well the statement doesn't state that If D then C, if C then B...so rules out
(B) If not B, then not C. the ---------C is very much dependent on B...If B then C.
(C) If not D, then not A. ---Well D is not dependent on A.
(D) If D, then E. --No mention in the statement
(E) If not A, then not D--Again A and D are not dependent

Source: 1000CRs of scoretop.com -- problem#7

7. 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.

[Guest]

I would choose C as my answer.
Here's why.
If C, then D has to be there.
If D isn't there, then C is surely not there. (If D does exist, C may or may not be there)
Similarly, if C isn't there, B isn't there. And if B isn't there, A isn't there.

[Guest]

Guest,
Deducting using your analogy, why is B not the answer?

According to you, if D did not exist, there is no way C can exist... then this should hold for choice B as well.

However, in your resp in paran, you said If NO D then C may or may not exist. If this were to be true, then the chain should follow upto A. So A may or may not exist...

Do you see why I am confused?

[Guest]

If not D, then no source that makes D is present (and of these multiple sources, C is one of them...and hence C is also not present). Similarly, if C is not present, then B is not present (even if it is only one source for C). And so goes for B and A.
The difference between (B) and (C) is that (B) mentions B as a source of C. But we do not know if B is the only source for C. C could be present, even if B is not present. Same thing goes for (D)
Does it make sense?

Another way...to make an educated guess:
Another way to look at this: B and A are pretty much the same. B and A are on left of C and D. So if you have to pick one answer, (B) and (E) cannot be the answers.
In (D), there is no mention of E in the premise. Hence we can rule out (D).
So now you're at (A) and (C). Nowhere in the premise it says that B can result onl from A. So on for C and B, and D and C (The last one....i.e. (A) is not so much of an educaed guess)

[dbernst]

Alright fellow GMATers, you're really making me reach deep into the GMAT bag of tricks for this one. In fact, I am actually reaching into my LSAT bag of tricks, since a question such as this is much more likely to show up on the LSAT than on the GMAT.

Whenever you're given a conditional statement as true, you can by definition also prove the negation of that statement in its inverse order. This is know as the contrapositive.

For example, let's examine the following statement:

If Justin Timberlake marries Britney Spears, then Christina Aguilera will be sad.
(Notice that this statement takes the form If A, then B.)

If the above statement is true, then, by definition, the following statement is also true:

If Christina Aguilera is not sad, then Justin Timberlake did not marry Britney Spears.
(Notice that this statement takes the form If Not B, then Not A.)

Applying this logic to our question, we have the following conditions:

If A, then B.
If Not B, then Not A
If B, then C.
IF Not C, then Not B
If C, then D.
If Not D, then Not C

Now, let's look at the answer choices:

(A) If D, then A. - Nothing in our conditions gives us an "If D" scenario
(B) If not B, then not C. - Our conditions say If Not B, then Not A; We know nothing about C
(C) If not D, then not A. - CORRECT. According to our conditions, If Not D, then Not C; IF Not C, then Not B; If Not B, then Not A
(D) If D, then E. - "E" does not even exist!!!
(E) If not A, then not D. - Nothing in our conditions gives us an "If not A" scenario

I love the LSAT!!! (Too bad I have no interest in the law!)
-dan

Source: 1000CRs of scoretop.com -- problem#7

7. 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.