You are observing a cocktail party that has seven guests. You notice that each guest shakes hands with one another once they meet, but never again. How many handshakes occurred during the cocktail party?
Many people quickly rush to the answer of 42 since 7(7-1) = 42. However, you must remember that once person A shakes hands with person B, person B does not re-shake hands with person A again. Therefore, the solution become (7(7-1))/2 = 21, or 1+2+3+4+5+6 = 21.