Find all School-related info fast with the new School-Specific MBA Forum

It is currently 29 Aug 2015, 01:40
GMAT Club Tests

Close

GMAT Club Daily Prep

Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track
Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

Events & Promotions

Events & Promotions in June
Open Detailed Calendar

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

  Question banks Downloads My Bookmarks Reviews Important topics  
Author Message
Current Student
User avatar
Joined: 11 May 2008
Posts: 560
Followers: 7

Kudos [?]: 72 [0], given: 0

There are 7 keys in a key ring. If two more keys are to be [#permalink] New post 05 Sep 2008, 07:39
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

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
Director
Director
avatar
Joined: 23 Sep 2007
Posts: 791
Followers: 5

Kudos [?]: 115 [0], given: 0

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
Manager
Manager
avatar
Joined: 15 Jul 2008
Posts: 207
Followers: 3

Kudos [?]: 37 [0], given: 0

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
VP
VP
User avatar
Joined: 05 Jul 2008
Posts: 1431
Followers: 36

Kudos [?]: 270 [0], given: 1

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
SVP
SVP
User avatar
Joined: 07 Nov 2007
Posts: 1824
Location: New York
Followers: 29

Kudos [?]: 590 [0], given: 5

Re: ring probability [#permalink] New post 05 Sep 2008, 16:32
Agree with D..
both logics are good.
_________________

Your attitude determines your altitude
Smiling wins more friends than frowning

Manager
Manager
avatar
Joined: 03 Jun 2008
Posts: 136
Schools: ISB, Tuck, Michigan (Ross), Darden, MBS
Followers: 2

Kudos [?]: 14 [0], given: 0

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!
_________________

-----------------------------------------------------------
'It's not the ride, it's the rider'

Re: ring probability   [#permalink] 05 Sep 2008, 23:26
Display posts from previous: Sort by

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

  Question banks Downloads My Bookmarks Reviews Important topics  


GMAT Club MBA Forum Home| About| Privacy Policy| Terms and Conditions| GMAT Club Rules| Contact| Sitemap

Powered by phpBB © phpBB Group and phpBB SEO

Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.