$PT\Leftrightarrow \frac{\frac{18(x-y)}{\frac{10(x+y)+400}{x+y}}}{2(x+y)}=0,3$
$\Leftrightarrow \frac{\frac{18(x-y)(x+y)}{10(x+y)+400}}{2(x+y)}=0,3$
Rút gọn tử mẫu cho $2(x+y)$
$PT \Leftrightarrow \frac{9(x-y)}{10(x+y)+400}=0,3$
$\Leftrightarrow 9(x-y)=3(x+y)+0,3.400$
$\Leftrightarrow 6x-12y=120$
$\Leftrightarrow x-2y=20 (đpcm)$