The gravitational force between two objects is F. If the masses of both objects are halved without changing distance between them, then the gravitational force would become
Gravitational Force F = G m1 m2 / r2
Where, m1, m2 are masses of two objects and r is distance between them.
When the masses are halved then F = 
= (Gm1m2/ r2 )
= F/4
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