1. mass of a planet is twice that of the earth and its radius is four times of the earth. find the value of g on its surface?
2. the gravitational force between two objects is 100 n. how should the distance between the objects be changed so that the force between them becomes 50 n?

1). Doubling the mass alone doubles the value of 'G'. 
Multiplying the radius by 4 alone multiplies the value of 'G' by 1/16 . 
The combined effect of both changes is to multiply the value of 'G' by (2/16) = 1/8. 

The acceleration of gravity on the surface of that mysterious planet is

                   (9.08) / 8  =  1.14 m/s² (rounded)

2). The gravitational force between two objects is inversely proportional  to
the square of the distance between them.  The force multiplies by 1/2 when
the distance between them grows by √2 = 1.414 (rounded).

If the gravitational force between two objects is initially 100N,  then it shrinks
to 50N when the initial distance between them becomes 41.4% greater.