Everyone shakes hands with everyone else in a room. Total number of handshakes is 78. Number of persons=?
Choose the correct choice
A) 13 B) 12 C) 11 D) 15 E) 16

Rationale: in a room of n people , the number of possible handshakes is c ( n , 2 ) or n ( n - 1 ) / 2 so n ( n - 1 ) / 2 = 78 or n ( n - 1 ) = 156 or n = 13 answer is ( a )
Answer: A