$$x+ \frac{ y }{ \frac{ 5-x- \frac{ y }{ 4 } }{ } }$$
$\frac{4y+20x-xy-4x^{2}}{20-y-4x}$
$\frac{16x^{2}+8xy-160x+y^{2}-24y+400}{\left(20-y-4x\right)^{2}}$
