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Senior Manager
Joined: 24 Mar 2011
Posts: 469
Location: Texas
Followers: 4

Kudos [?]: 54 [1] , given: 20

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Question Stats:

50% (01:42) correct 50% (01:15) wrong based on 6 sessions
A certain business school has 500 students, and the law school at the same university has 800 students. Among these students, there are 30 sibling pairs consisting of 1 business student and 1 law student. If 1 student is selected at random from both schools, what is the probability that a sibling pair is selected?

3/40,000
3/20,000
3/4,000
9/400
6/130

In the above Q, what the green color text means? i think it means 2 students (pair) to be selected from the gang since it is mentioned in the red that sibling pair is selected but i just thought that green text is missing 'each'.
Math Forum Moderator
Joined: 20 Dec 2010
Posts: 2047
Followers: 128

Kudos [?]: 902 [0], given: 376

agdimple333 wrote:
A certain business school has 500 students, and the law school at the same university has 800 students. Among these students, there are 30 sibling pairs consisting of 1 business student and 1 law student. If 1 student is selected at random from both schools, what is the probability that a sibling pair is selected?

3/40,000
3/20,000
3/4,000
9/400
6/130

In the above Q, what the green color text means? i think it means 2 students (pair) to be selected from the gang since it is mentioned in the red that sibling pair is selected but i just thought that green text is missing 'each'.

Good point agdimple333:
I prefer the wordings of the following link:
probability-57997.html

The use of word "both" and "pair" indeed creates some ambiguity as to whether we need to choose 2 students from each class OR 1 student from both classes combined.

Let's see using both scenarios:
If 2 students, 1 from each class, need to be selected.
P(Pick Sibling Pair)=(30/500)*(1/800)=30/400000= 3/40000

If just 1 student needs to be selected,
P(Pick from Sibling Pairs)=(30+30)/(500+800)=60/1300=6/130
_________________
Senior Manager
Joined: 24 Mar 2011
Posts: 469
Location: Texas
Followers: 4

Kudos [?]: 54 [0], given: 20

thank you and the correct answer is A.
Manager
Joined: 08 Sep 2010
Posts: 172
Followers: 0

Kudos [?]: 15 [0], given: 18

fluke,

don't you have your definitions reversed? you say 2 students and select only 30 and then you say 1 student and select (30+ 30).
I might be mistaken.

Let's see using both scenarios:
If 2 students, 1 from each class, need to be selected.
P(Pick Sibling Pair)=(30/500)*(1/800)=30/400000= 3/40000

If just 1 student needs to be selected,
P(Pick from Sibling Pairs)=(30+30)/(500+800)=60/1300=6/130
_________________

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Current Student
Joined: 26 May 2005
Posts: 571
Followers: 18

Kudos [?]: 105 [0], given: 13

Hi Puneet,
What Fluke has done is right..
there are 30 pairs of siblings( total 60; 30 in each class)
so selecting one is 500 students is 30/500
now that we have selected 1 sibling - selection of his/her partner is pretty easy
we need to get only that partner out of 30 siblings in 800 students
so its 1/800

together 30/400000 = 3/40000

selecting one student can be done ;

30 pairs = 60 student .
selecting one is 60C1
total 800+500 = 1300
one of 1300 is 1300C1
hence prob= 60c1/1300c1 = 6/130
Manager
Joined: 08 Sep 2010
Posts: 172
Followers: 0

Kudos [?]: 15 [0], given: 18

thanks Sudhir..got it
_________________

My will shall shape the future. Whether I fail or succeed shall be no man's doing but my own.

If you like my explanations award kudos.

Director
Joined: 01 Feb 2011
Posts: 770
Followers: 14

Kudos [?]: 81 [0], given: 42

1 student from the both the schools translates to 1 from each school.

agdimple333 wrote:
A certain business school has 500 students, and the law school at the same university has 800 students. Among these students, there are 30 sibling pairs consisting of 1 business student and 1 law student. If 1 student is selected at random from both schools, what is the probability that a sibling pair is selected?

3/40,000
3/20,000
3/4,000
9/400
6/130

In the above Q, what the green color text means? i think it means 2 students (pair) to be selected from the gang since it is mentioned in the red that sibling pair is selected but i just thought that green text is missing 'each'.
Manager
Joined: 14 Feb 2011
Posts: 70
Followers: 1

Kudos [?]: 1 [0], given: 2

Hi, guys. So what is the right answer?

My method.
I understand the question that we take 1 person from each group.
P(person has a sibling in another group)=30/500
P(2nd group person is sibling)=1/800.

Multiply=30/40 000 or 3/4000

Am I right?
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