If initially the distance between two bodies is $r$ and their masses be $M_1$ and $M_2$, then the force of gravitation be $F$. If this distance is increased to two times, then the force would become

According to the universal law of gravitation, the force between two bodies is inversely proportional to the square of the distance between the two bodies. 
$\mathit{F\; \alpha }\frac{1}{r^2}$
So, when the distance is increased to two times, the new force will be
$F^{\text{'}}\mathit{\alpha }\frac{1}{{\left(2r\right)}^2}$
$\Rightarrow F^{\text{'}}= \frac{1}{4r^2}$
$\Rightarrow F^{\text{'}}= \frac{F}{4}$