Question 981206
Before the additions, x girls, y boys.
{{{highlight_green(x/(x+y)=0.12)}}}
{{{x=0.12x+0.12y}}}
{{{100x=12x+12y}}}
{{{88x=12y}}}
{{{22x=3y}}}



The additions;
{{{highlight_green((x+15)/(x+15+y+5)=0.15)}}}
{{{(x+15)/(x+y+20)=0.15}}}
{{{x+15=0.15(x+y+20)}}}
{{{100x+1500=15x+15y+15*20}}}
{{{85x+1500=15y+15*20}}}
{{{85x=15y+15*20-1500}}}
{{{85x-15y=300-1500}}}
{{{15y-85x=1200}}}
{{{3y-17x=240}}}


System of equations
{{{system(22x=3y,3y-17x=240)}}}
Both of which have the term {{{3y}}}, making finding of x somewhat easy to do.
{{{22x=240+17x}}}
{{{5x=240}}}
{{{x=24*2*5/5}}}
{{{highlight(x=48)}}}-----original count of the girls.
-
{{{22x=3y}}}
{{{22x/3=y}}}
{{{y=22*48/3}}}
{{{y=22*16}}}
{{{highlight(y=352)}}}------original count of the boys.