g = number of girls;
b= number of boys
we know that: g= 6+2b
and that: g+b= 156 kids in total
so we may write g+b=(6+2b)+b=6+3b
but g+b= 156
so 6+3b = 156 => 3b= 156-6=150 => b=150/3=50 => b = 50 (number of boys)
g= 6+2b= 6+2 x 50= 106 => g =106 (number of girls)