If 13 persons meet at a reunion and each person shakes hands exactly once with each of the others, what is the total number of handshakes? Answers:
A) 10•9•8•7•6•5•4•3•2•1 B) 10•10 C) 78 D) 45 E) 36
We got #13 people who shake each other's hands once ==&gt; a pair of 2   13!/11!2! = 13*12 / 2*1= 
<gadget id="calculator">13*12 / 2*1</gadget>
<output>78</output>
78.   Hence answer 78.
<result>C</result>
If 13 persons meet at a reunion and each person shakes hands exactly once with each of the others, what is the total number of handshakes?