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Kaplan Test - Combo problem

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by adi_800 » Sun Oct 02, 2011 1:16 am
How many circles can be drawn on the line segment if each circle is subject to the rule that the endpoints of one of its diameters must be any two of the points A , B , C , D , E , F , G , H , J , K , L , and M ?

36
66
72
78
121

Cannot post the figure as of now... Imagine a straight line and it has points marked from A to M in the order given above...
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by adi_800 » Sun Oct 02, 2011 1:20 am
Oh Realized that one should use combination formula to solve this one..
OA is B

nCk = n!/k!(n-k)! where n = 12 and k = 2
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by gmatboost » Mon Oct 03, 2011 10:38 am
I recommend that everyone memorize the fact that nC2 = n(n-1)/2, for any value of n.
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by Brent@GMATPrepNow » Mon Oct 03, 2011 12:21 pm
To learn how to quickly calculate combinations such as 12C2 in your head, you can watch video #17 at https://www.gmatprepnow.com/module/gmat-counting (it's free)

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
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