Solution:
Let the total number of members be x.
So xC2 = 190.
Or x(x-1)/2 = 190.
Or x(x-1) = 380.
Solving you get x = 20.
The correct answer is A.
Reverse Combinatorics Problem - GMAT Prep
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How did you move from xC2 to x(x-1)/2?Rahul@gurome wrote:Solution:
Let the total number of members be x.
So xC2 = 190.
Or x(x-1)/2 = 190.
Or x(x-1) = 380.
Solving you get x = 20.
The correct answer is A.
(I am really terrible at Combinatorics)
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Rahul@gurome
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xC2 is x!/{(x-2)!*2!}.
Now x! is x(x-1)(x-2)....2*1 = x(x-1)(x-2)!.
So x!/{(x-2)!*2!} is {x(x-1)(x-2)!}/{(x-2)!*2!} = x(x-1)/2! = x(x-1)/2.
Now x! is x(x-1)(x-2)....2*1 = x(x-1)(x-2)!.
So x!/{(x-2)!*2!} is {x(x-1)(x-2)!}/{(x-2)!*2!} = x(x-1)/2! = x(x-1)/2.
Rahul Lakhani
Quant Expert
Gurome, Inc.
https://www.GuroMe.com
On MBA sabbatical (at ISB) for 2011-12 - will stay active as time permits
1-800-566-4043 (USA)
+91-99201 32411 (India)
Quant Expert
Gurome, Inc.
https://www.GuroMe.com
On MBA sabbatical (at ISB) for 2011-12 - will stay active as time permits
1-800-566-4043 (USA)
+91-99201 32411 (India)
