Model for the Combined variation  is   
 
Equation to find the mass of the rocket in terms of F and d is :

<h3>What is Force ?</h3>
A force is the quantity that change the motion of the object. Force is that influence which can cause any object having some mass, to change it's position or to accelerate from it's original position. 
Force (F) and mass (m) of any object are  directly proportional to each other. More the mass of the object , more the force will require to accelerate it. 
Mathematically, we can write the equation of force as :
F = m . a 
where , m = mass of the object
              a = constant value
In the statement given, 
As, force of gravity varies directly with its mass 
Therefore, F= k m , where k is a constant
Also force is inversely varies with square of distance d
Thus, F = k / d² 
Combining both equations :
 
   
and the equation of mass in terms of F and d can be written as :

To know more about Force and it's equation, visit : 
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