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The probability

by TuanNguyen87 » Wed Jun 08, 2011 4:57 pm
A certain junior class has 1,000 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 propability that the 2 students will be a sibling pair?

A. 3/40,000
B. 1/3,600
C. 9/2,000
D. 1/60
E. 1/15

Answer: A

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by Anurag@Gurome » Wed Jun 08, 2011 5:22 pm
TuanNguyen87 wrote:A certain junior class has 1,000 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 propability that the 2 students will be a sibling pair?

A. 3/40,000
B. 1/3,600
C. 9/2,000
D. 1/60
E. 1/15

Answer: A

Thanks a lot everyone!
Required probability = No. of favorable outcome/ Total no. of outcomes
Total no. of outcomes = 800C1 * 1000C1
There are 60 sibling pairs, so no. of favorable outcomes = 60
Therefore, Required probability = = 60/(800*1000) = 3/40,000

The correct answer is A.
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by g.shankaran » Wed Jun 08, 2011 5:22 pm
TuanNguyen87 wrote:A certain junior class has 1,000 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 propability that the 2 students will be a sibling pair?

A. 3/40,000
B. 1/3,600
C. 9/2,000
D. 1/60
E. 1/15

Answer: A

Thanks a lot everyone!
If 1 student is to be selected at random from each class, what is the propability that the 2 students will be a sibling pair?

junior class = 1000 stu
senior class = 800 stu

There are 60 sibling pairs. so each class has 60 students who are siblings.

selecting 1 student from junior class who is a sibling is 60C1 = 60
selecting 1 student from junior class = 1000C1 = 1000

selecting 1 student from senior class who is a sibling is 1 ( We have already selected a student from junior class. so there will be only one student in the senior class who is a sibling of the student selected from junior class)
selecting 1 student from senior class = 800C1 = 800


so, probability = 60/1000 * 1/800 = 3/40000 Hence A
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by pemdas » Wed Jun 08, 2011 5:22 pm
probability that we select any (good) student from the first (any) group is either 60/1000 or 60/800
probability that the second student we select from the group will also be good (sibling) is either 1/800 or 1/1000

i used two alternatives above to show that it does not matter from which group we select first

60/1000 * 1/800
60/800 * 1/1000

both are giving 3/40,000 a
TuanNguyen87 wrote:A certain junior class has 1,000 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 propability that the 2 students will be a sibling pair?

A. 3/40,000
B. 1/3,600
C. 9/2,000
D. 1/60
E. 1/15

Answer: A

Thanks a lot everyone!
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by cans » Wed Jun 08, 2011 8:08 pm
total J=1000,total S=800
60 sibling pairs(1J,1S)
Total pairs = 1 from J and 1 from S = 1000*800
prob = 60/800000 = 3/40000
IMO A
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by phanideepak » Wed Jun 08, 2011 8:22 pm
Since there are 60 sibling pairs we have 60 students in Junior class and 60 students in senior class who are siblings.

probability of selecting a sibling pair = (60/1000)(1/800) = 3/40000
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by [email protected] » Wed Jun 08, 2011 8:54 pm
OA A - 3/40000
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by bubbliiiiiiii » Wed Jun 08, 2011 11:06 pm
Thanks for the post.

I chose C as I have incorrectly counted the number of siblings instead of sibling pairs. :)

Thanks for the learning.
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by Jeff@TargetTestPrep » Thu Feb 22, 2018 5:05 pm
TuanNguyen87 wrote:A certain junior class has 1,000 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 propability that the 2 students will be a sibling pair?

A. 3/40,000
B. 1/3,600
C. 9/2,000
D. 1/60
E. 1/15
The probability of selecting any one sibling from the 60 sibling pairs in the junior class is 60/1000. Once that person is selected, the probability of selecting his or her sibling from the senior class is 1/800; thus, the probability of a selecting a sibling pair is:

60/1000 x 1/800 = 3/50 x 1/800 = 3/40000

Alternatively, the probability of selecting any one sibling from the 60 sibling pairs in the senior class is 60/800. Once that person is selected, the probability of selecting his or her sibling from the junior class is 1/1000; thus, the probability of a selecting a sibling pair is:

60/800 x 1/1000 = 3/40 x 1/1000 = 3/40000

Answer: A

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