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# Issue with an alternate method in Problem Solving

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Intern
Joined: 31 May 2017
Posts: 2
Issue with an alternate method in Problem Solving  [#permalink]

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13 Nov 2017, 07:45
00:00

Difficulty:

(N/A)

Question Stats:

100% (00:00) correct 0% (00:00) wrong based on 1 sessions

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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

What's wrong with below method?

The set of people with 1 sibling is 4 and that of 2 siblings is 3. We select one person from each set by calculating 4C1 and 3C1 respectively.
The number of favourable outcomes then becomes 4C1*3C1.
The number of all possible outcomes is 7C2.
Probability = (4C1*3C1)/7C2 = 4/7.

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Math Expert
Joined: 02 Sep 2009
Posts: 50009
Re: Issue with an alternate method in Problem Solving  [#permalink]

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13 Nov 2017, 08:09
sunilrawat wrote:
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

What's wrong with below method?

The set of people with 1 sibling is 4 and that of 2 siblings is 3. We select one person from each set by calculating 4C1 and 3C1 respectively.
The number of favourable outcomes then becomes 4C1*3C1.
The number of all possible outcomes is 7C2.
Probability = (4C1*3C1)/7C2 = 4/7.

Discussed here: in-a-room-filled-with-7-people-4-people-have-exactly-87550.html In case of any questions please post in that thread.

--== Message from the GMAT Club Team ==--

THERE IS LIKELY A BETTER DISCUSSION OF THIS EXACT QUESTION.
This discussion does not meet community quality standards. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.

_________________
Re: Issue with an alternate method in Problem Solving &nbs [#permalink] 13 Nov 2017, 08:09
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