A composite disc is built from a solid disc and a concentric, thick-walled hoop, as shown below. The inner disc (solid) has mass 4 kg and radius 2 m. The outer disc (thick-walled) has mass 5 kg, inner radius 2 m, and outer radius 3 m. Calculate the moment of inertia of this composite disc about a central axis perpendicular to the discs.

• A. 40.5 kg•m²
• B. 53 kg•m²
• C. 67 kg•m²
• D. 70.5 kg•m²

Answer: (A)