A solid wooden cylinder has radius 5 cm and height 12 cm…

Step 1. The cylinder. $\pi \times 25\ \text{cm}^2 \times 12\ \text{cm} = 300\pi\ \text{cm}^3$.

Step 2. The hollow. $\dfrac{1}{3} \times 300\pi\ \text{cm}^3 = 100\pi\ \text{cm}^3$.

Step 3. What is left. $300\pi - 100\pi = 200\pi\ \text{cm}^3$, which is $628.3\ \text{cm}^3$.

Step 4. The percentage. $\dfrac{100\pi}{300\pi} \times 100 = 33.3$ percent.