[tex]\displaystyle\bf\\\frac{10y+z}{y} =0,5\\\\\\\frac{10y}{y} +\frac{z}{y} =0,5\\\\\\10+\frac{z}{y} =0,5\\\\\\\frac{z}{y} =0,5-10\\\\\\\boxed{\frac{z}{y} =-9,5}[/tex]