If 11 persons meet at a reunion and each person shakes hands exactly o

The correct answer is D.

If 11 persons meet at a reunion and each person shakes hands exactly once with each of the others, what is the total number of handshakes?

1st person will have 10 handshakes, 2nd will have 9, ....10th will have 1 and 11th will have 0
= 10+9+8+7+6+5+4+3+2+1 = 55

if we generalize this: if there are n people then there will be n(n-1)/2 handshakes.

Ans.D) 55

JJo I think you meant n(n+1)/2 handshakes.

Hi All,

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...

10+9+8+7+6+5+4+3+2+1+0

For a total of 55

Final Answer:

GMAT assassins aren't born, they're made,
Rich

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

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

Our case : 11*10/2 = 55 . Answer is (D).

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