We know that, the gravitational force

F = G. m₁m₂/r²              …(i)

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(i) If the mass of one object is doubled,

then   F₁ = G. (2m₁)m₂/r² 

F₁ = 2. Gm₁m₂/r²

F₁ = 2F

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Therefore, the gravitational force will be two times.

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(ii) If the distance between the objects is doubled.

then F₂ =  G. (m₁)m₂/(2r)² 

F₂ = 1/4 . Gm₁m₂/r²

F₂ = F/4

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Therefore, the gravitational force will be one-fourth.

If the distance between the objects is tripled.

then F₃ =  G. (m₁)m₂/(3r)² 

F₃ = 1/9 . Gm₁m₂/r²

F₃ = F/9

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Therefore, the gravitational force will be 1/9 times.

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(iii) If the masses of both objects are double

 F₁ = G. (2m₁)(2m₂)/r² 

F₁ = 4. Gm₁m₂/r²

F₁ = 4F

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Therefore the gravitational force will be four times.