1. **State the problem:** We need to find the area of a triangle with a base of 20 meters and an altitude (height) of 45 meters. 2. **Formula for the area of a triangle:** $$\text{Area} = \frac{1}{2} \times \text{base} \times \text{height}$$ 3. **Substitute the given values:** $$\text{Area} = \frac{1}{2} \times 20 \times 45$$ 4. **Calculate the product:** $$\text{Area} = \frac{1}{2} \times 900$$ 5. **Simplify the multiplication:** $$\text{Area} = 450$$ 6. **Final answer:** The area of the triangle is **450 square meters**.