The acceleration due to gravity on the surface of earth is g . If...

The acceleration due to gravity is related to the mass and the radius of the Earth by the formula:

$g = \dfrac{GM}{{R}^{2}} ⇒ g ∝ \dfrac{1}{{R}^{2}} . . . \left(1\right)$

Thus, from equation (1), it follows that

$\dfrac{{g}_{2}}{{g}_{1}} = {R}_{1}^{2}{R}_{2}^{2} = {R}_{1}^{2}{\left(\dfrac{{R}_{1}}{2}\right)}^{2} = 4 ⇒ {g}_{2} = 4g$