1. **State the problem:** Find the volume of a rectangular prism with dimensions 5 units wide, 3 units tall, and 8 units long. 2. **Formula:** The volume $V$ of a rectangular prism is given by the formula: $$V = \text{length} \times \text{width} \times \text{height}$$ 3. **Apply the formula:** Substitute the given dimensions: $$V = 8 \times 5 \times 3$$ 4. **Calculate step-by-step:** $$V = 8 \times 5 = 40$$ $$V = 40 \times 3 = 120$$ 5. **Interpretation:** The volume of the prism is 120 cubic units, meaning it can hold 120 unit cubes inside. **Final answer:** $$\boxed{120}$$