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by karim.nazmy » 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. :)

https://www.gmatprepnow.com/module/gmat- ... /video/762
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by [email protected] » 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:

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

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by Matt@VeritasPrep » 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. :)

https://www.gmatprepnow.com/module/gmat- ... /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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