Question 1030934
x=boys in A
y=girls in A
y-4=x or -x=y-4
Class A has y-4+y students or 2y-4

2x+y is B
x+3 boys and y-5 girls.
But in B x+3=2(y-5)
x=2y-13
x-y-4 rom above.  
-x=-y+4 add them
0=y-9
y=9
x=5
Class A has 5 boys and 9 girls
Class B has 8 boys and 4 girls, which is twice as many.