If 10 people meet and shakes hand with each other.
First person will shakes hand with other 9,
second person had already shakes hand with 1st person and shakes hand with other 8,
So, the total no. of handshakes= 9+8+7+6+5+4+3+2+1=45
Shakes The other way to calculate this is by the formula n(n+1)/2 where n is the no. of shake hands by very first person and that is 9. So, the total no. of handshakes=9(9+1)/2 = 9(10)/2 =90/2 =45