A particular straight line passes through origin and a point whose abscissa is double of ordinate of the point. The equation of such straight line is

At a point ,$\mathrm{Abscissa} =2\times \mathrm{ordinate}$

$\Rightarrow x_0=2y_0$

so, co-ordinate of the point is $\left(2\mathrm{a}, \mathrm{a}\right)$ from some value of a.

equation of a straight line : $\mathrm{y}=\mathrm{mx}+\mathrm{c}$

as it passes through origin, $\mathrm{c}=0$

for the given point, 

$$\begin{gathered} \mathrm{a}=2\mathrm{ma} \\ \Rightarrow \mathrm{m}=\frac{1}{2} \end{gathered}$$
$\Rightarrow \mathrm{y}=\frac{\mathrm{x}}{2}$