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?
+37159 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
