When placed in a water of density 997 kgm3 a hollow sphere

Question

a) When placed in a water of density 997 kg/m³, a hollow sphere of inner radius 10cm and outer radius 11cm floats with half of its volume below the level of the free surface of the water. Compute the density of the material the sphere is made.

b) When placed in oil this sphere floats 60% of its volume below the level of the free surface of the oil. Compute the density of the oil.