Answer:
The volume of the hollow ball will be equal to the difference between the volume of a sphere with the outer radius and the volume of a sphere with the inner radius.
Remember that the volume of a sphere of radius R is given by:
V = (4/3)*pi*R^3
where pi = 3.14
Then if R is the outer radius, and r is the inner radius, the volume of the hollow ball is:
V = (4/3)*pi*( R^3 - r^3)
In this case, we have:
R = 200m
r = 100m
Then the volume of the hollow ball is:
V = (4/3)*3.14*( (200m)^3 - (100m)^3)
V = 29,306,666.67 m^3
Now, if this hollow ball is made of iron, we want to find the mass.
Remember the relation:
Mass = Density*Volume.
We know that the density of the iron is:
D = 7300 kg/m^3
Then the mass of this hollow ball is:
M = (7300 kg/m^3)*(29,306,666.67 m^3) = 213,938,666,666.6 kg