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GRE Prep Club Team Member
Joined: 20 Feb 2017
Posts: 2508
Given Kudos: 1053
GPA: 3.39
A certain junior class has 1,000 students and a certain senior class
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20 Nov 2021, 01:07
20 Nov 2021, 01:07
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A certain junior class has 1,000 students and a certain senior class has 800 students. Among these students, there are 60 siblings 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 at will be a sibling pair?
A. 3/40,000
B. 1/3,600
C. 9/2,000
D. 1/60
E. 1/15
A. 3/40,000
B. 1/3,600
C. 9/2,000
D. 1/60
E. 1/15
Retired Moderator
Joined: 10 Apr 2015
Posts: 6218
Given Kudos: 136
Re: A certain junior class has 1,000 students and a certain senior class
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20 Nov 2021, 06:10
20 Nov 2021, 06:10
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GeminiHeat wrote:
A certain junior class has 1,000 students and a certain senior class has 800 students. Among these students, there are 60 siblings 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 at will be a sibling pair?
A. 3/40,000
B. 1/3,600
C. 9/2,000
D. 1/60
E. 1/15
A. 3/40,000
B. 1/3,600
C. 9/2,000
D. 1/60
E. 1/15
One option is to apply probability rules.
So, for two siblings to be selected, 2 things must happen: we must select a junior who has a senior sibling AND the senior selected must be the sibling of the selected junior.
We get P(junior with sibling AND selected senior is sibling to selected junior)= P(junior with sibling) x P(selected senior is sibling to selected junior)
P(junior with sibling): there are 1000 juniors and 60 of them have senior siblings. So, P(junior with sibling)=60/1000
P(selected senior is sibling to selected junior): Once the junior has been selected, there is only 1 senior (out of 800 seniors) who is the sibling to the selected junior. So, P(selected senior is sibling to selected junior)= 1/800
So, the probability is (60/1000)x(1/800) = 3/40,000
The answer is A
Cheers,
Brent
gmatclubot
Re: A certain junior class has 1,000 students and a certain senior class [#permalink]
20 Nov 2021, 06:10
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