LSAT Prep question HELP!!!!

DrPepper237
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Joined: Tue Jul 26, 2011 3:20 pm

LSAT Prep question HELP!!!!

Postby DrPepper237 » Wed Aug 03, 2011 9:39 am

I'm really confused... This is linear game 7 from the McGraw-Hill's Conquering the LSAT Logic Games book (2nd edition).

I am confused by the answer for question 4. Here are the constraints.

There are 6 swimmers (Anna through Frank).

Ben and Dana swim in consecutive lanes.
Anna and Evan swim in consecutive lanes.
Chris and Frank swim in consecutive lanes.
If Evan swims in a lower-numbered lane than Ben, then Frank swims in lane 4.

B<>D
A<>E
C<>F
E<B-->F4 contrapositive not F4 --> B<E

THE QUESTION: If Anna swims in lane 6, then which of the following must be true?
(A) Dana swims in the lane numbered just below Ben's lane.
(B) Frank swims in a lower numbered lane than Dana.
(C) Chris swims in a higher-numbered lane than Anna.
(D) Evan swims in a higher-numbered lane than Dana.
(E) Frank swims in a lower-numbered lane than Chris.

So based on the contrapositive of "not F4 --> B<E", I chose answer (B): Frank swims in a lower-numbered lane than Dana, because if A is in lane 6, then E has to be in lane 5. Therefore, E is automatically considered to be in a higher numbered lane than B could possibly be, which means F can't be in lane 4, which means F has to be in lanes 1 or 2. The book says the answer is (D). So.... it seems to me that both answers are right since the constraints/contrapositive dictate that F must be in a lower-numbered lane than Dana and E must be in a higher numbered lane than Dana.

HEEEELLLLPPPP!!!!!

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annyong
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Joined: Tue Jul 05, 2011 10:17 am

Re: LSAT Prep question HELP!!!!

Postby annyong » Wed Aug 03, 2011 9:46 am

DrPepper237 wrote:Therefore, E is automatically considered to be in a higher numbered lane than B could possibly be, which means F can't be in lane 4, which means F has to be in lanes 1 or 2.

Why can't he be in 3?

DrPepper237
Posts: 7
Joined: Tue Jul 26, 2011 3:20 pm

Re: LSAT Prep question HELP!!!!

Postby DrPepper237 » Wed Aug 03, 2011 10:06 am

annyong wrote:
DrPepper237 wrote:Therefore, E is automatically considered to be in a higher numbered lane than B could possibly be, which means F can't be in lane 4, which means F has to be in lanes 1 or 2.

Why can't he be in 3?


E can't be in lane 3 because E needs to be in a consecutive lane with A, which means if A is in lane 6 then E needs to be in lane 5.

F can't be in lane 3, because if E is in lane 5, he is in a higher-numbered lane than B, which means F can't be in lane 4 according to the contrapositive "not F4 --> B<E". If F isn't in lane 4, he can't be in lane.... ooooo.....hmmmm.....

whoops.... i see it now.

omg i'm so screwed for this test....

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KevinP
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Re: LSAT Prep question HELP!!!!

Postby KevinP » Thu Aug 04, 2011 1:09 am

Drop McGraw-Hill's conquering the LSAT and go with Powerscore/Manhattan for LG.

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WhoIsDonDraper
Posts: 55
Joined: Tue Jul 05, 2011 2:19 am

Re: LSAT Prep question HELP!!!!

Postby WhoIsDonDraper » Thu Aug 04, 2011 1:28 am

DrPepper237 wrote:I'm really confused... This is linear game 7 from the McGraw-Hill's Conquering the LSAT Logic Games book (2nd edition).

I am confused by the answer for question 4. Here are the constraints.

There are 6 swimmers (Anna through Frank).

Ben and Dana swim in consecutive lanes.
Anna and Evan swim in consecutive lanes.
Chris and Frank swim in consecutive lanes.
If Evan swims in a lower-numbered lane than Ben, then Frank swims in lane 4.

B<>D
A<>E
C<>F
E<B-->F4 contrapositive not F4 --> B<E

THE QUESTION: If Anna swims in lane 6, then which of the following must be true?
(A) Dana swims in the lane numbered just below Ben's lane.
(B) Frank swims in a lower numbered lane than Dana.
(C) Chris swims in a higher-numbered lane than Anna.
(D) Evan swims in a higher-numbered lane than Dana.
(E) Frank swims in a lower-numbered lane than Chris.

So based on the contrapositive of "not F4 --> B<E", I chose answer (B): Frank swims in a lower-numbered lane than Dana, because if A is in lane 6, then E has to be in lane 5. Therefore, E is automatically considered to be in a higher numbered lane than B could possibly be, which means F can't be in lane 4, which means F has to be in lanes 1 or 2. The book says the answer is (D). So.... it seems to me that both answers are right since the constraints/contrapositive dictate that F must be in a lower-numbered lane than Dana and E must be in a higher numbered lane than Dana.

HEEEELLLLPPPP!!!!!


Lose the book you're using and buy either Manhattan or Powerscore Logic Games. Your main problem is that you committed a negation of the sufficient condition fallacy. Just because the "E<B" part of the conditional is negated, that still doesn't mean that F cannot go in position 4. F can go wherever he wants at this point. All you can validly conclude so far is what you said about A and E.

KapTeacherBobby
Posts: 9
Joined: Wed Jul 27, 2011 1:15 pm

Re: LSAT Prep question HELP!!!!

Postby KapTeacherBobby » Thu Aug 04, 2011 7:39 pm

DrPepper237 wrote:So based on the contrapositive of "not F4 --> B<E", I chose answer (B): Frank swims in a lower-numbered lane than Dana, because if A is in lane 6, then E has to be in lane 5.


Review that part of your logic...You are definitely correct in deducing that E must be in lane 5...does this allow us to draw any conclusions about where in the sequence F and D are?

What we can see is that E is in 5 so EVERYONE (except for A) must be swimming in a lower-numbered lane.

HellOnHeels
Posts: 548
Joined: Sun Jul 31, 2011 8:19 pm

Re: LSAT Prep question HELP!!!!

Postby HellOnHeels » Thu Aug 04, 2011 8:28 pm

DrPepper237 wrote:Ben and Dana swim in consecutive lanes.
Anna and Evan swim in consecutive lanes.
Chris and Frank swim in consecutive lanes.
If Evan swims in a lower-numbered lane than Ben, then Frank swims in lane 4.

B<>D
A<>E
C<>F
E<B-->F4 contrapositive not F4 --> B<E

THE QUESTION: If Anna swims in lane 6, then which of the following must be true?
(A) Dana swims in the lane numbered just below Ben's lane.
(B) Frank swims in a lower numbered lane than Dana.
(C) Chris swims in a higher-numbered lane than Anna.
(D) Evan swims in a higher-numbered lane than Dana.
(E) Frank swims in a lower-numbered lane than Chris.

If A is in 6, and E must swim next to A, then E is in 5. So...

1 2 3 4 5 6
_ _ _ _ E A

You cannot use the last bit of info (if E<B) because it can't happen; E will be in a higher lane than B. So all you know is...
1 2 3 4 5 6
_ _ _ _ E A

when answering your question. All you know is E/A swim in a higher numbered lane than B/C/D/F. That rules out all options except (D). At least that is how I would answer that question. I'm still new at this :oops:




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