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This question doesn't require any special math formulas - you just need to take some notes and do a bit of arithmetic.

We're told that 11 people will each shake hands with one another. We're asked how many handshakes actually occur:

I'm going to name the 11 people ABCDE FGHIJ and K

A shakes the hands of the 10 other people. B has ALREADY shook hands with A, so B shakes 9 'new' hands. C has ALREADY shook hands with A and B, so C shakes 8 'new hands. Etc.

You should notice that with each new person, the number of 'new' handshakes decreases by 1. This gives us...

its the sum from 1 to 10, coz the person can't shake hands with himself then each next to him loses 1 chance to shake hands because he's shaken hands with the one counted:

10+9+8+......+1=(10+1)*10/2=55

this shortcut applies to other questions the like, such as games between two teams blablabla

Re: If 11 persons meet at a reunion and each person shakes hands exactly o [#permalink]

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23 Mar 2016, 08:00

santorasantu : This can be made simple. Derive your own formula wherever needed. Case 1.Two men would need only a single hand shake. Case 2.Three men would need three hand shakes. Your formula must satisfy above two cases. Hence it is n(n-1)/2. Case 1. 2*1/2 = 1 Case 2. 3*2/2 = 3

Re: If 11 persons meet at a reunion and each person shakes hands exactly o [#permalink]

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05 Aug 2017, 17:06

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