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Probability

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by Sant » Sat May 31, 2008 5:32 am
The answer is 1/5 right? Not 1/5! as mentioned above.
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by molt_llest » Sat May 31, 2008 8:01 am
Can you explain how do you know that?

I check the answer and I think there is a !
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by jasonc » Sat May 31, 2008 7:03 pm
1/4 of the squares have a 2/15 chance of forming a rectangle
1/4 of the squares have a 4/15 chance of forming a rectangle
1/2 of the squares have a 1/5 chance of forming a rectangle

Therefore, when you add up the weighted chances, you get 1/5
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by Stuart@KaplanGMAT » Sat May 31, 2008 9:02 pm
Honestly, I'd just brute force this one.

We can make 3 rectangles in each row (e.g. 1/2, 2/3, 3/4 in the first row) for a total of 12 horizontal rectangles and 3 rectangles in each column (e.g. 1/5, 5/9 and 9/13 in the first column) for a total of 12 vertical rectangles. So, we can make 24 total 2 number rectangles.

There are 16C2 total pairs we can pick:

16!/2!14! = 16*15/2 = 8*15 = 120

Probabilty = #desired/total # = 24/120 = 1/5
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by molt_llest » Sun Jun 01, 2008 3:30 am
Thank you, I understand it now.

I guess there is a problem with the signs in the answers.
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