BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

combination 2

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
User avatar
Master | Next Rank: 500 Posts
Posts: 101
Joined: Tue Oct 27, 2009 8:19 am
Thanked: 4 times
Followed by:2 members

combination 2

by g.shankaran » Tue May 31, 2011 10:31 am
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?

5/21

3/7

4/7

5/7

16/21
Top
Source: — Problem Solving |
Legendary Member
Posts: 1448
Joined: Tue May 17, 2011 9:55 am
Location: India
Thanked: 375 times
Followed by:53 members

by Frankenstein » Tue May 31, 2011 10:37 am
Hi,
You can go through the following recent post.

https://www.beatthegmat.com/friends-prob ... tml#370376

Cheers!
Top
User avatar
Legendary Member
Posts: 1309
Joined: Mon Apr 04, 2011 5:34 am
Location: India
Thanked: 310 times
Followed by:123 members
GMAT Score:750

by cans » Tue May 31, 2011 11:59 pm
3 people have exactly 2 siblings in room. let those 3 be a,b,c
Thus a,b,c are siblings
Now 4 people have exactly 1 sibling.. let people be d,e,f,g
and let d,e are siblings and f,g are siblings.
thus we can select 2 people from these 3 groups.
if from d,e & f,g => 2*2=4ways
if from d,e & a,b,c => 2*3=6ways
if from f,g & a,b,c => 2*3=6ways
total 16
Now in 7C2 ways we can select any 2 persons. 7C2 = 21
prob = 16/21
IMO E
Top
Post new topic Post Reply
• Page 1 of 1