1. ## quick question

Consider a group of 20 people. If everyone shakes hands with everyone else, how many handshakes take place?
thanks

2. Originally Posted by mauro21pl
Consider a group of 20 people. If everyone shakes hands with everyone else, how many handshakes take place?
thanks
That is number of combinations possible with 2 people out of 20.
${{20}\choose 2}=\frac{20\cdot 19}{2}$