1. The problem statement, all variables and given/known data A uniform solid sphere of mass m and radius a has moment of inertia 2/5ma2 about any diameter. Material is removed from the sphere to make a concentric spherical cavity of radius a/2. What is the mass of the resulting hollow ball ? Show that its moment of inertia about a diameter is (31/80)ma2 What does it mean that moments of inertia are additive and why are they so ? 2. Relevant equations I have calculated the mass of the hollow ball to be (7/6)πρa3 3. The attempt at a solution I've been attempting to solve this question by calculating the moment of inertia of the material removed from the the sphere and subtracting that from the moment of inertia given for a solid sphere, but this approach doesn't give me the correct coefficient of 31/80, I've checked my maths many times. I am just wondering if this is the correct way of doing this problem?