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# Because I doubt the correctness of the OA, so I post this

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Manager
Joined: 30 Mar 2009
Posts: 233
Because I doubt the correctness of the OA, so I post this [#permalink]

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Updated on: 12 May 2009, 05:19
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Because I doubt the correctness of the OA, so I post this question:

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

Pls explain mates

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Originally posted by Minheequang on 12 May 2009, 04:38.
Last edited by Minheequang on 12 May 2009, 05:19, edited 1 time in total.
Intern
Joined: 02 Mar 2009
Posts: 42
Location: Austin
Re: If ... then ... [#permalink]

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12 May 2009, 04:59
IMO C.
SVP
Joined: 07 Nov 2007
Posts: 1738
Location: New York
Re: If ... then ... [#permalink]

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12 May 2009, 05:52
Minheequang wrote:
Because I doubt the correctness of the OA, so I post this question:

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

Pls explain mates

If A, then B
means
A : sufficient condition
B : necessary condition.

A --> B
B -->C
C---> D

from the above it is clear A --> D

Contrapostive of A --> D
is
not D --> not A

(A) If D, then A
D-->A ( Mistaken reversal)

(B) If not B, then not C
not B--> not C ( Mistaken negation)

(D) If D, then E

E is unknown, we can't deduce this equation

(E) If not A, then not D

not A --> not D (Mistaken negation)
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Manager
Joined: 30 Mar 2009
Posts: 233
Re: If ... then ... [#permalink]

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12 May 2009, 06:56
x2suresh wrote:
Minheequang wrote:
Because I doubt the correctness of the OA, so I post this question:

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

Pls explain mates

If A, then B
means
A : sufficient condition
B : necessary condition.

A --> B
B -->C
C---> D

from the above it is clear A --> D

Contrapostive of A --> D
is
not D --> not A

(A) If D, then A
D-->A ( Mistaken reversal)

(B) If not B, then not C
not B--> not C ( Mistaken negation)

(D) If D, then E

E is unknown, we can't deduce this equation

(E) If not A, then not D

not A --> not D (Mistaken negation)

So, I guess wrong, the OA is right but your explanation makes me even more confusing !!!

This kind of reasoning method seems strange to me but actually normal. However, because I oversimplify it then I get the wrong choice (B)
Now, I get the whole picture, lets check it:
It's about cause, effect relationship (your statement of sufficient - necessary condition makes me confusing)
If A, then B --> A: cause, B: effect

(A) If D, then A --> this is completely wrong because it reverses the role. So eliminate it
(B) If not B, then not C --> lets analyzing it:
If B is not the cause, then we have 3 possibilities for the cause:
- If A is the cause, then the effect will be B, and then C will appear as a effect of cause B --> it counters this choice
- If C is the cause, then the effect can't be C --> it supports this choice
- If D is the cause, then the effect can't be C --> it supports this choice
so, this choice is not always right (2 right but 1 wrong) --> it's wrong choice
(C) If not D, then not A --> analyzing: also 3 possibilities:
- If A is the cause, then the effect can't be A --> it supports this choice
- If B is the cause, then the effect can't be A --> it supports this choice
- If C is the cause, then the effect can't be A --> it supports this choice
So, it's always right --> best choice
(D) If D, then E --> E is not mentioned --> wrong
(E) If not A, then not D --> analyzing: also 3 possibilities:
- If B is the cause, then the effect will be C, and then D will appear as a effect of cause C --> it counters this choice
- If C is the cause, then the effect will be D --> it counters this choice
- If D is the cause, then the effect can't be D --> it supports this choice
2 wrong, 1 right --> wrong choice
Manager
Joined: 08 Apr 2009
Posts: 91
Re: If ... then ... [#permalink]

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12 May 2009, 10:20
IMO C too
Director
Joined: 25 Oct 2006
Posts: 592
Re: If ... then ... [#permalink]

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12 May 2009, 12:14
Minheequang wrote:
Because I doubt the correctness of the OA, so I post this question:

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 => No sure it could be b or C
(B) If not B, then not C => Not sure, because if C comes from Y; in that case, B may not be there but C should
(C) If not D, then not A = > YES. Just put some meaningful text in place of A and D. if TV is plugged(A), Program can be viewed(D). If program cannot be viewed, TV is NOT plugged.
(D) If D, then E = > OOS
(E) If not A, then not D = > if B is there, D could be there [TV is plugged but you did not switch on the remote, so program cannot be viewed]
Pls explain mates

Probably, you could eliminate other possibilities.
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Manager
Joined: 25 Mar 2009
Posts: 52
Re: If ... then ... [#permalink]

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15 May 2009, 01:17
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

(A) & (C) are equivalent clauses: out both
(D) If D then E, not mentioned

2 answers left (B) & (E), I am stuck right here

P/S all of my good-math-friends chose B when were asked within 15s, they said this is a kind of logical math (clause math)

--== Message from GMAT Club Team ==--

This is not a quality discussion. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.
Re: If ... then ...   [#permalink] 15 May 2009, 01:17
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# Because I doubt the correctness of the OA, so I post this

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