Question

How many handshakes are possible at a party of 65 people if everyone shakes hands once?

Answer 1

You must calculate how many ways can 2 people be selected from 65.
The formula is:
n! / r! * (n-r)! =
65! / 2! * 63! =
65*64*63! / 2 * 63! =
65*64 / 2 =
2,080 handshakes

Source:
http://www.1728.org/combinat.htm