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Probability Question

This topic has 2 expert replies and 0 member replies
karim.nazmy Newbie | Next Rank: 10 Posts Default Avatar
Joined
17 Oct 2015
Posted:
2 messages

Probability Question

Post Sat Oct 17, 2015 2:09 pm
I need help to understand why my answer is incorrect to this problem when i use combinations. There is a total of 9C2 combinations which are 36. To get 2 of the same color, there are 2 green combinations, 1 red and 1 yellow. Therefore, answer would be (2+1+1)/36 = 4/36= 2/9.

Plz let me know what im doing wrong here. Smile

http://www.gmatprepnow.com/module/gmat-probability/video/762

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Post Sat Oct 17, 2015 3:46 pm
Hi karim.nazmy,

You'll likely get more of a response if you post your Quant question (including the entire prompt and answer choices) here:

http://www.beatthegmat.com/problem-solving-f6.html

GMAT assassins aren't born, they're made,
Rich

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Contact Rich at Rich.C@empowergmat.com

GMAT/MBA Expert

Post Fri Oct 30, 2015 12:28 am
karim.nazmy wrote:
I need help to understand why my answer is incorrect to this problem when i use combinations. There is a total of 9C2 combinations which are 36. To get 2 of the same color, there are 2 green combinations, 1 red and 1 yellow. Therefore, answer would be (2+1+1)/36 = 4/36= 2/9.

Plz let me know what im doing wrong here. Smile

http://www.gmatprepnow.com/module/gmat-probability/video/762
You're on the right track:

Green pairs: (4 choose 2) = 6
Red pairs: (3 choose 2) = 3
Yellow pairs: (2 choose 2) = 1

so there are 6 + 3 + 1 = 10 valid pairs.

There are also (9 choose 2) = 36 total pairs.

That makes our probability 10/36, or 5/18.

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