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There are 7 keys in a key ring. If two more keys are to be

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There are 7 keys in a key ring. If two more keys are to be [#permalink] New post 05 Sep 2008, 07:39
There are 7 keys in a key ring. If two more keys are to be added in the ring at random, what is the probability that two keys will be adjacent?
(A) 1/8
(B) 1/6
(C) 1/5
(D) 1/4
(E) 1/3
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Re: ring probability [#permalink] New post 05 Sep 2008, 08:28
arjtryarjtry wrote:
There are 7 keys in a key ring. If two more keys are to be added in the ring at random, what is the probability that two keys will be adjacent?
(A) 1/8
(B) 1/6
(C) 1/5
(D) 1/4
(E) 1/3



D

7 keys in the ring, if you add 1 more, you will have 8 keys. The ring is circular, so you have 8 slots for putting in the next key, but the next key can be to the left or to the right of the 8th key. so 2/8=1/4
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Re: ring probability [#permalink] New post 05 Sep 2008, 08:30
arjtryarjtry wrote:
There are 7 keys in a key ring. If two more keys are to be added in the ring at random, what is the probability that two keys will be adjacent?
(A) 1/8
(B) 1/6
(C) 1/5
(D) 1/4
(E) 1/3


1/4.

Total number of ways to arrange 9 keys in circular permutation = 8!

Consider the 2 keys in question as unit. So you have 8 units which can be arranged in 7! ways. The pair can be arranged in 2 ways. So total ways in which the pair always stays together = 2*7!

Prob = (2*7!) / 8! = 1/4
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Re: ring probability [#permalink] New post 05 Sep 2008, 14:21
number of ways for arranging n people in a circle is (n-1)!

Total 9 keys in a ring 8! ways

Adjacent means keep the two rings as one unit and the 8 rings can be arranged in 7! ways. the 2 rings can be arranged in 2 ways in between them

Ans is ( 2X 7!) / 8! which is 1/4
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Re: ring probability [#permalink] New post 05 Sep 2008, 16:32
Agree with D..
both logics are good.
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Re: ring probability [#permalink] New post 05 Sep 2008, 23:26
bhushangiri wrote:
arjtryarjtry wrote:
There are 7 keys in a key ring. If two more keys are to be added in the ring at random, what is the probability that two keys will be adjacent?
(A) 1/8
(B) 1/6
(C) 1/5
(D) 1/4
(E) 1/3


1/4.

Total number of ways to arrange 9 keys in circular permutation = 8!

Consider the 2 keys in question as unit. So you have 8 units which can be arranged in 7! ways. The pair can be arranged in 2 ways. So total ways in which the pair always stays together = 2*7!

Prob = (2*7!) / 8! = 1/4


Nice explanation!
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Re: ring probability   [#permalink] 05 Sep 2008, 23:26
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