average weight of 12 students in a class is 30kg. two boys leave the class and two boys whose avg weight is 25kg join the class. if the new average of the class is 30 5/6kg, what is the avg weight of the boys who left the class?

• 20 kg
  
  Let the average weight of the boys who left the class be 'x' kg then
  Total weight after joining two new boys = 12*30 - 2x + 2*25 = 12*185/6
  410 - 2x = 370
  2x = 40
  x = 20
  Therefore, the avg weight of the boys who left the class=20 kg.
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