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24 Aug 2016, 08:28
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A certain junior class has 600 students and a certain senior class has 800 students. Among these students, there are N sibling pairs, consisting of a Junior and a Senior. What is the value of N?
(1) If a student is selected from each class at random, the probability that the students do not form a sibling pair is 4799/4800.
(2) If a student is selected from each class at random, the probability that exactly ONE of these students is a member of a sibling pair is 1/4.
Source: Old Manhattan Material
(1) If a student is selected from each class at random, the probability that the students do not form a sibling pair is 4799/4800.
(2) If a student is selected from each class at random, the probability that exactly ONE of these students is a member of a sibling pair is 1/4.
Source: Old Manhattan Material
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Senthil1981
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24 Aug 2016, 10:17
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yosita18
A certain junior class has 600 students and a certain senior class has 800 students. Among these students, there are N sibling pairs, consisting of a Junior and a Senior. What is the value of N?
(1) If a student is selected from each class at random, the probability that the students do not form a sibling pair is 4799/4800.
(2) If a student is selected from each class at random, the probability that exactly ONE of these students is a member of a sibling pair is 1/4.
Source: Old Manhattan Material
(1) If a student is selected from each class at random, the probability that the students do not form a sibling pair is 4799/4800.
(2) If a student is selected from each class at random, the probability that exactly ONE of these students is a member of a sibling pair is 1/4.
Source: Old Manhattan Material
I could get the first statement , need help in the solving the second statement.
Consider 1)If a student is selected from each class at random, the probability that the students do not form a sibling pair is 4799/4800.
so the probability that they form a sibling pair is \(1- \frac{4799}{4800} = \frac{1}{4800}\)
So \(\frac{N}{600} * \frac{N}{800} = \frac{1}{4800}\) . Sufficient Hence A or D
24 Aug 2016, 17:57
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yosita18
A certain junior class has 600 students and a certain senior class has 800 students. Among these students, there are N sibling pairs, consisting of a Junior and a Senior. What is the value of N?
(1) If a student is selected from each class at random, the probability that the students do not form a sibling pair is 4799/4800.
(2) If a student is selected from each class at random, the probability that exactly ONE of these students is a member of a sibling pair is 1/4.
Source: Old Manhattan Material
(1) If a student is selected from each class at random, the probability that the students do not form a sibling pair is 4799/4800.
(2) If a student is selected from each class at random, the probability that exactly ONE of these students is a member of a sibling pair is 1/4.
Source: Old Manhattan Material
Statement 1: 1/4800= N/480000
Therefore, N=100
Sufficient
Statement 2: No. of groups formed with sibling from Junior class = siblings from junior class * non-siblings from senior class = N*(800-N)
No. of groups formed with sibling from Senior class = siblings from senior class * non-siblings from junior class = N*(600-N)
Therefore (N(800-N)+N(600-N))/600*800 = 1/4
solving this gives quadratic equation: N^2-700N+60000=0
So N =100 or 600
Insufficient
Ans A
Archived Topic
Hi there,
This topic has been closed and archived due to inactivity or violation of community quality standards. No more replies are possible here.
Where to now? Join ongoing discussions on thousands of quality questions in our Data Sufficiency (DS) Forum
Still interested in this question? Check out the "Best Topics" block above for a better discussion on this exact question, as well as several more related questions.
Thank you for understanding, and happy exploring!
