(C) 200 N
Explanation:
The acceleration due to gravity on earth [tex]g_{E}[/tex] is given by
[tex]g_{E} = G\dfrac{M_{E}}{R_{E}^{2}}[/tex]
where [tex]G[/tex] = universal gravitational constant
[tex]\:\:\:\:\:\:\:\:\:\:\:\:M_{R}[/tex] = mass of the earth
[tex]\:\:\:\:\:\:\:\:\:\:\:\:R_{E}[/tex] = radius of the earth
Planet Krypton has twice the mass of earth and 3 times the radius so its acceleration due to gravity [tex]g_{K}[/tex] is
[tex]g_{K} = G\dfrac{M_{K}}{R_{K}^{2}}[/tex]
[tex]\:\:\:\:\:\: = G\dfrac{(2M_{E})}{(3R_{E})^{2}}[/tex]
[tex]\:\:\:\:\;\:= (\dfrac{2}{9})G\dfrac{M_{E}}{R_{E}^{2}}[/tex]
or
[tex]g_{K} = (\dfrac{2}{9})\:g_{E}[/tex]
If we multiply both sides by Superman's mass, we get his weights on both planets:
[tex]mg_{K} = (\dfrac{2}{9})\:(mg_{E})[/tex]
[tex]W_{K} = (\dfrac{2}{9})\:W_{E} = (\dfrac{2}{9})(900\:N)= 200\:N[/tex]
