there are 12 friends. each greeted each other by shaking hands. how many handshakes took place.

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- February 15th 2011, 07:16 AMMoniMiniHandshakes
there are 12 friends. each greeted each other by shaking hands. how many handshakes took place.

- February 15th 2011, 07:40 AMBAdhi
Use principle of counting

- February 15th 2011, 07:42 AMMoniMini
- February 15th 2011, 07:48 AMBAdhi
why do you say it takes a long time to get the answer, if there are n objects and 2 slots for objects, no of permutations are equal to n(n-1). Since there changing two objects in a group(a,b and b,a) doesn't make a difference divide it by 2

- February 16th 2011, 04:46 AMHallsofIvy
What that tells me is that you do not know what the "principle of counting" is! No, it does not mean "write out all the possible handshakes and count them up".

There are 12 friends and each shakes hands with each of the 11 others. The "principle of counting" says there are 12*11 "handshakes". But that counts the handshake between "Andy" and "Barbara" twice- once for Andy and once for Barbara. So, as BAdhi says, divide by 2.