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A certain junior class has 1000 students and a certain

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by BTGmoderatorDC » Thu Aug 16, 2018 9:08 pm

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A certain junior class has 1000 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 will be a sibling pair?

A. 3/40000
B. 1/3600
C. 9/2000
D. 1/60
E. 1/15

OA A

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by Keith@ThePrincetonReview » Fri Aug 17, 2018 6:32 am
BTGmoderatorDC wrote:A certain junior class has 1000 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 will be a sibling pair?

A. 3/40000
B. 1/3600
C. 9/2000
D. 1/60
E. 1/15

OA A

Source: GMAT Prep
There are 60 sibling pairs.
The probability of selecting a sibling from the junior class is 60/1,000.
The probability of selecting the other member of the sibling pair from the senior class is 1/800.
The probability that both students selected will be a sibling pair is 60/1,000 * 1/800 = 3/40,000.
The correct answer is choice A.
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by Jeff@TargetTestPrep » Mon Aug 20, 2018 10:45 am
BTGmoderatorDC wrote:A certain junior class has 1000 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 will be a sibling pair?

A. 3/40000
B. 1/3600
C. 9/2000
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

Jeffrey Miller
Head of GMAT Instruction
[email protected]

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by swerve » Mon Aug 20, 2018 12:55 pm
Probability = number of ways to select 1 junior and 1 senior such that they make a sibling pair / number of ways to select 1 junior and 1 senior.
= 60 / (1000*800)
= 60 / 800000
= 3 / 400000

Or option A. Regards!
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