a sphere is constructed of two concentric pieces. The inner part is a solid sphere of radiu 10.0 cm made of a material with density 4000 kd/m^3. The outer part is a spherical shell with inner radiu 10.0 cm and outer radius 20.0 cm. The outer shell has a density 9000 kg/m^3.
a) What is the mass of this sphere?
b) What is the average density of this sphere?