V_e and V_p denote the escape velocities from the earth and another planet having twice the radius and the same mean density as the earth, then

V_e = V_p/2

$\frac{\mathrm{v}_{\mathrm{e}}(\text { earth })}{\mathrm{v}_{\mathrm{p}} \text { (planet) }}=\frac{\mathrm{v}_{\mathrm{e}}}{\mathrm{v}_{\mathrm{p}}}=\frac{1}{2} ∴ \mathrm{v}_{\mathrm{e}}=\frac{\left(\mathrm{v}_{\mathrm{p}}\right)}{2}$