Cześć!
a)
[tex]x=-20\\\\(\frac{1}{4}x-5)(\frac{1}{5}x-4)=(\frac{1}{4}\cdot(-20)-5)\cdot(\frac{1}{5}\cdot(-20)-4)=\\\\=(-5-5)\cdot(-4-4)=(-10)\cdot(-8)=\boxed{80}[/tex]
b)
[tex]x=12, \ y=-2\\\\\frac{x-xy^2}{2y}=\frac{12-12\cdot(-2)^2}{2\cdot(-2)}=\frac{12-12\cdot4}{-4}=\frac{12-48}{-4}=\frac{-36}{-4}=\boxed9[/tex]
