Question

the mean weight of 60 students in a class is 40 kg if the mean weigth of boys is30kg then find the number of boys and girls

Answer 1

 The mean weight of 60 students = 40 kg
ie     The sum of 60 student weight
         ----------------------------------------   = 40
                              60
              sum of the 60 student           = 40 * 60   = 2400 kg
   the weight of (boys + girls)               = 2400
                            
any one information ie either boys or girls number then only we can solve this sum