1. **State the problem:** We have a wall with dimensions length = 10 m, thickness = 20 cm = 0.2 m, and height = 1.2 m. We need to find the cost of cementing five sides of this wall at the rate of 25 per square meter. 2. **Formula and explanation:** The surface area to be cemented includes the two large faces (length × height), the two small faces (thickness × height), and the top face (length × thickness). The bottom face is not cemented. 3. **Calculate areas:** - Area of two large faces: $$2 \times (10 \times 1.2) = 2 \times 12 = 24 \text{ m}^2$$ - Area of two small faces: $$2 \times (0.2 \times 1.2) = 2 \times 0.24 = 0.48 \text{ m}^2$$ - Area of top face: $$10 \times 0.2 = 2 \text{ m}^2$$ 4. **Total surface area to be cemented:** $$24 + 0.48 + 2 = 26.48 \text{ m}^2$$ 5. **Calculate cost:** $$\text{Cost} = 26.48 \times 25 = 662 \text{ (units)}$$ **Final answer:** The cost of cementing the five sides of the wall is 662.