The lines $10x+5y-20=0$ and $y=-2x+20$ are

$10x+5y-20=0$ is compare with $ax+by+c=0$

$m_1=\frac{-10}{5}=-2$

$y=-2x+20$ is compare with $y=m_{2}x+c$

$m_2=-2$

$m_1=m_2=-2$

$\therefore$ Given lines are parallel lines