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by chaitanya.mehrotra » Sun Jul 24, 2011 12:07 pm
In a room filled with 7 people, 4 people have exactly 1 sibling in the room and 3 people have exactly 2 siblings in the room. If two individuals are selected from the room at random, what is the probability that those two individuals are NOT siblings?
5/21
3/7
4/7
5/7
16/21

OA after some discussion. Let me know the time it took you to solve this question . For me it took 4:14 min to solve this .
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by knight247 » Sun Jul 24, 2011 2:12 pm
Keeping in mind at all times that the total number of ppl in the room is 7
4 ppl have exactly one sibling meaning two sibling pairs
3 ppl have exactly two siblings meaning a group of 3 siblings i.e. 3 ppl who are siblings.


7C2 is the number of possible outcomes

Desired outcomes
one person from 1st sibling pair.one person from 2nd sibling pair+one person from sibling trio.one person from 1st sibling pair+one person from sibling trio.one person from 2nd sibling pair
Which is
(2C1.2C1+3C1.2C1+3C1.2C1)/7C2
(2.2+3.2+3.2)/21
4+6+6/21=16/21 hence E
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