A certain junior class has 1000 students and a certain : PS Archive
Check GMAT Club Decision Tracker for the Latest School Decision Releases http://gmatclub.com/AppTrack

 It is currently 22 Jan 2017, 03:39

### 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

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

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# A certain junior class has 1000 students and a certain

Author Message
Intern
Joined: 12 May 2005
Posts: 32
Followers: 0

Kudos [?]: 11 [0], given: 0

A certain junior class has 1000 students and a certain [#permalink]

### Show Tags

05 Jan 2008, 13:23
1
This post was
BOOKMARKED
00:00

Difficulty:

(N/A)

Question Stats:

86% (01:00) correct 14% (01:02) wrong based on 49 sessions

### HideShow timer Statistics

This topic is locked. If you want to discuss this question please re-post it in the respective forum.

A certain junior class has 1000 students and a certain senior class has 800 students. Among these students, there are 60 sibling pairs, each consisting of 1 junior and 1 senior.If 1 student is to be selected at random from each class, what is the probability that the 2 students selected will be a sibling pair?
A: 3/40000
B: 1/3600
C:9/2000
D: 1/60
E:1/15
OA is A.
Can someone please explain this problem in detail?
Logic says since there are 60 sibling pairs:
There are 30 seniors and 30 juniors
So probability is p= 30/1000* 30/800.
Why is this wrong?
Intern
Joined: 25 Sep 2007
Posts: 22
Followers: 0

Kudos [?]: 16 [0], given: 0

### Show Tags

05 Jan 2008, 13:41
60 pairs of siblings = 120 people; 60 juniors and 60 seniors; Probability of selecting a junior out of the class of 1000 people is 60/1000. Probability of selecting his/her sibling out of the class of seniors is 1/800 (since only one person can be his/her sibling);

So we have 60/1000 * 1/800 = 3/40000
Intern
Joined: 12 May 2005
Posts: 32
Followers: 0

Kudos [?]: 11 [0], given: 0

### Show Tags

05 Jan 2008, 13:46
Oops. I got the question wrong then. Thanks that makes sense.
Manager
Joined: 27 Oct 2008
Posts: 185
Followers: 2

Kudos [?]: 144 [0], given: 3

### Show Tags

27 Sep 2009, 09:54
A certain junior class has 1000 students and a certain senior class has 800 students. Among these students, there are 60 sibling pairs, each consisting of 1 junior and 1 senior.If 1 student is to be selected at random from each class, what is the probability that the 2 students selected will be a sibling pair?
A: 3/40000
B: 1/3600
C:9/2000
D: 1/60
E:1/15

Soln: Total no. of ways of choosing two people, one from each class is 800 * 1000.
Of these total outcomes, there are only 60 pairs which are sibling pairs. Hence answer is
= 60/(800 * 1000)
= 3/40000

ans is A
Re: Probability   [#permalink] 27 Sep 2009, 09:54
Display posts from previous: Sort by