There are 10 married couples at a party. At the start of the party, every person shakes hands once with every other person except his or her spouse. How many handshakes are there?
20 people have to shake hands with 18 other folks
20 x 18 = 360 but each handshake involves TWO people, so divide by 2 360/2 = 180 handshakes
All twenty of them shake hands with 18 people. 20 * 18 = a lot of handshakes