If then

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sarthak: Senior | Next Rank: 100 Posts; Posts: 75; Joined: Tue Jan 19, 2010 10:43 am

by sarthak » Thu Mar 18, 2010 10:40 am

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

OA later

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Source: Beat The GMAT — Critical Reasoning | Thu Mar 18, 2010 10:40 am

mircealitoiu: Newbie | Next Rank: 10 Posts; Posts: 6; Joined: Tue Nov 10, 2009 10:38 pm

by mircealitoiu » Thu Mar 18, 2010 5:13 pm

IMO A

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sarthak: Senior | Next Rank: 100 Posts; Posts: 75; Joined: Tue Jan 19, 2010 10:43 am

by sarthak » Thu Mar 18, 2010 9:05 pm

OA is C

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[email protected]: Senior | Next Rank: 100 Posts; Posts: 33; Joined: Tue Jan 19, 2010 11:15 pm; Location: India; Thanked: 2 times

by [email protected] » Thu Mar 18, 2010 10:33 pm

sarthak wrote:OA is C

Can you please explain.

IMO it is B This is a 'must be true' question. hence, the answer must be from the information contained in the original statement.

Very confusing. Can you pls give the original explanation.

I am very doubtful whether this is a GMAT type question.

GMAT GMAT Whatcha gona do.. Whatcha gona do when I come for you...

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sarthak: Senior | Next Rank: 100 Posts; Posts: 75; Joined: Tue Jan 19, 2010 10:43 am

by sarthak » Thu Mar 18, 2010 10:51 pm

This is actually a LSAT problem . Which is supposed to be 1 step tougher than GMAT in CR. So its good to practice couple of problems from there to keep prepared.

And alas ... I only have the OA ... no explanation.IMO B and E

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Phirozz: Master | Next Rank: 500 Posts; Posts: 232; Joined: Sun Feb 28, 2010 10:47 pm; Thanked: 10 times

by Phirozz » Fri Mar 19, 2010 2:00 am

sarthak wrote: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

OA later

if A implies B, then negation B implies negation A. So C

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mj41: Senior | Next Rank: 100 Posts; Posts: 56; Joined: Wed Feb 17, 2010 3:26 am

by mj41 » Fri Mar 19, 2010 3:32 am

I think it is C because from the sequence it shows that A through the process will always lead to D.

Hence If D does not exist then A was never there. Because if A were there then it would lead to B, then C and then D. But if there is no D then there was no A.

I know it is real confusing but hope I made sense.

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sudi760mba: Senior | Next Rank: 100 Posts; Posts: 85; Joined: Wed Aug 13, 2008 2:21 am; Thanked: 1 times

by sudi760mba » Sat Mar 20, 2010 3:36 pm

mj41 wrote:I think it is C because from the sequence it shows that A through the process will always lead to D.

Hence If D does not exist then A was never there. Because if A were there then it would lead to B, then C and then D. But if there is no D then there was no A.

I know it is real confusing but hope I made sense.

Can't the same argument be made of E? Why not E?

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kevincanspain: GMAT Instructor; Posts: 613; Joined: Thu Mar 22, 2007 6:17 amFollowed by: 64 members; Location: madrid; Thanked: 171 times; GMAT Score:790

by kevincanspain » Sat Mar 20, 2010 3:47 pm

Make sure you know the difference between a sufficient condition

If A, then B (For B to be true, it is sufficient that A is true)

and a necessary condition

If not A, then not B. (B is true only if A is true. This is not to say that B is true if A is true) For B to be true, A must be true. That A is true does not necessarily mean that B is true

Note that if A implies B (A is sufficient condition for B), not B implies not A (B is a necessary condition for A)

Think of the following

A- is a GMAT student
B- speaks at least some English
C- is a human being
D- is a mammal
E- loves Dorothy Parker's short stories

Kevin Armstrong
GMAT Instructor
Gmatclasses
Madrid

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kstv: Legendary Member; Posts: 610; Joined: Fri Jan 15, 2010 12:33 amFollowed by: 2 members; Thanked: 47 times

by kstv » Sat Mar 20, 2010 6:42 pm

Tried depicting the logic in a diagram invloving concentric circles
where A is the innermost circle and D the outer most.
If A then B so A is a subset of B so the circle A is completely inside B
if B then C so B is subset of C so A is inside B which inturn is inside C
if C then D so C is a subset of D.
Tried labelling them as Kevin has done in his post. With A as those who are taking GMAT and D as Mammals.
Finding the correct answer C is easier.

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