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

Redeem

Selection Problem

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Senior | Next Rank: 100 Posts
Posts: 97
Joined: Wed Sep 01, 2010 11:11 am
Thanked: 1 times
Followed by:2 members

Selection Problem

by akshatgupta87 » Fri Apr 22, 2011 12:37 pm
Q.) 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
Someone explain..
Top
Source: — Problem Solving |
User avatar
GMAT Instructor
Posts: 3225
Joined: Tue Jan 08, 2008 2:40 pm
Location: Toronto
Thanked: 1710 times
Followed by:614 members
GMAT Score:800

by Stuart@KaplanGMAT » Fri Apr 22, 2011 12:42 pm
Hi!

This question has been discussed many times - I ran a search on "4 people have exactly 1 sibling" and hit a ton of threads.

Here's one in which I explain the question:

https://www.beatthegmat.com/sibling-prob ... 46183.html

and there are many others if you don't click with that one!
Image

Stuart Kovinsky | Kaplan GMAT Faculty | Toronto

Kaplan Exclusive: The Official Test Day Experience | Ready to Take a Free Practice Test? | Kaplan/Beat the GMAT Member Discount
BTG100 for $100 off a full course
Top
Newbie | Next Rank: 10 Posts
Posts: 3
Joined: Mon Apr 18, 2011 2:52 am

by sh.nada » Sun Apr 24, 2011 5:04 am
Thanks Stuart

However, i have tried another approach to solve this question but i failed miserably.

i multiplied the probability of selecting the first NOT sibling by the Probability of selecting the second not sibling as follows

4/7 * 3/6 = 2/7 (not one of the answer choices)

kindly help me to understand how to approach such questions

Thank you
Top

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 16207
Joined: Mon Dec 08, 2008 6:26 pm
Location: Vancouver, BC
Thanked: 5254 times
Followed by:1268 members
GMAT Score:770

by Brent@GMATPrepNow » Sun Apr 24, 2011 5:33 am
sh.nada wrote:Thanks Stuart

However, i have tried another approach to solve this question but i failed miserably.

i multiplied the probability of selecting the first NOT sibling by the Probability of selecting the second not sibling as follows

4/7 * 3/6 = 2/7 (not one of the answer choices)

kindly help me to understand how to approach such questions

Thank you
The big problem with your approach is that everyone in the room is a sibling (there are 2 pairs of siblings and one trio of siblings).
So, the first person you select must be someone's sibling.
In your solution you begin by finding the probability of selecting the first NOT sibling, which you say is 4/7, but this probability must be zero, since it's impossible to select someone who is not a sibling.

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
Image
Top
Post new topic Post Reply
• Page 1 of 1