What happens to the force between two objects, if 

(i) The mass of one object is doubled. 

(ii) The distance between the objects is doubled and tripled? 

(iii) The masses of both objects are doubled.

We have the force due to gravity given by,

$F=G\frac{Mm}{R^2}$

(i) The mass of one object is doubled

$F^1=G\frac{M\left(2m\right)}{R^2}$ = $2F$

So, if one mass say is doubled, then the force gets doubled
 

(ii) The distance between the objects is doubled and tripled

$F^1=G\frac{Mm}{{\left(2R\right)}^2}$= $\frac{F}{4}$
 

If the distance is tripled, then the force will be,

$F^1=G\frac{Mm}{{\left(3R\right)}^2}$= $\frac{F}{9}$
 

(iii) The masses of both objects are doubled

$F^1=G\frac{\left(2M\right)\left(2m\right)}{R^2}$ = $4F$