Question

1) A body weighs 30 kg on the surface of the earth. How much would it weigh on the surface of the planet whose mass is 1/9 the mass of the earth and radius is half that of earth.

Answer 1

mass m = 30 kg  on Earth
weight = m g = m G M / R²
          G = universal gravitational constant
          M = mass of Earth
          R = Radius of Earth

mass of the body remains same on the surface of other planets.
gravity on a planet = g' = G M' / R' ²
                         M' = M/9        and  R' = R/2
     =>  g ' = G M/R * 4/9 = 4 g / 9
  
weight on the planet = m g' = 4/9 m g  = 4/9 * 30 kg = 40/3  kg

Answer 2

f=mg
f= 700 GM/r^2

f= 700 g m/7/(r/2)^2
f = 400 GM