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Seven People handshakes-Math riddle

29.Math Riddles
If seven people meet each other and each shake hands only once with each of the others, how many handshakes happened? 
  • A.14
  • B.49
  • C.42
  • D.21
    • Explanation :  
      6+5+4+3+2+1 = 21.
      One of the seven shakes hands with the other six.
      One of the other six shakes hands with all but the first one.
      And so on, down to the last handshake.