1. **Problem Statement:** Calculate the volume of the prism in 14b, which has a triangular cross-sectional area $A = 20$ mm² and length $12$ mm. 2. **Formula Used:** The volume $V$ of a prism is given by the formula: $$V = A \times l$$ where $A$ is the area of the cross-section and $l$ is the length of the prism. 3. **Explanation:** Since the cross-section is triangular with area $20$ mm², and the prism extends along the length of $12$ mm, multiply these values to find the volume. 4. **Calculation:** $$V = 20 \times 12 = 240$$ 5. **Answer:** The volume of the prism is $240$ cubic millimeters (mm³).