Register    Login    Search    Rss Feeds

Forums Home » Ask An Instructor » GMAT Math » Manhattan GMAT CAT Math


 Page 1 of 1 [ 2 posts ] 



  Print view Print view Previous topic Previous topic | Next topic Next topic
 

In a room filled with 7 people

Author Message
jonizatt
 Post subject: In a room filled with 7 people
 Post Posted: Wed Sep 28, 2011 6:58 pm 
Offline
Course Students


Posts: 1
In a room filled with 7 people, 4 people have exactly 1 sibling in the room and 3 people have exactly 2 siblings in the room. If two individuals are selected from the room at random, what is the probability that those two individuals are NOT siblings?

a 5/21
b 3/7
c 4/7
d 5/7
e 16/21

In the solution it says that 4 people have 1 sibling, and this would account for 2 sibling relationships. (I don't understand this logic)

It also says that you multiply (7*6)/2 to find that there are 21 different ways to choose two people from the room. I don't understand where they got these #'s.

A more detailed explanation would help out a lot.

Thanks
Top 
StaceyKoprince
 Post subject: Re: In a room filled with 7 people
 Post Posted: Fri Nov 11, 2011 10:20 pm 
Offline
ManhattanGMAT Staff


Posts: 6077
Location: San Francisco
Please search before you post - then you don't have to wait for a reply if the problem has already been posted and discussed! This problem has been posted multiple times before; here's the thread with the largest number of replies;

siblings-problem-combinatorics-t9048.html?hilit=In%20a%20room%20filled%20with%207%20people

If you have additional questions, post in the existing thread. Thanks!

_________________
Stacey Koprince
Instructor
Director of Online Community
ManhattanGMAT
Top 
Display posts from previous:  Sort by  
 
 Page 1 of 1 [ 2 posts ] 





Who is online

Users browsing this forum: No registered users and 0 guests

 
 

 
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to: