<PRE><B>Question 964924</B>
let the no of girls in class A= x
the no of girls in class B= y
7 girls shifted from A to B
after shifting  in class A= x-7
                in class B= y+7
given condition is no of girls are interchanged 
      so   in class A -  x-7 = y
           in class B -  y+7 = x
  this gives equation  x=y+7 ---------(1)
now after shifting 4 girls from B to A 

   in class A = x+4
   in class B =y-4
  given condition is  the number of girls in class A would be twice the original number of girls in class B
      x+4= 2*y  --------(2)
     substitute (1) in (2)
        y+7+4 = 2*y
      2*y - y = 11
      y=11
   x= 11+7 =18
 in class A   girls = 18
              boys = 35-18=17
 in class B   girls = 11
              boys = 35-11=24

  HENCE SOLVED 
</PRE>