1. **State the problem:** You need to find how many 20 kg bags of premixed concrete are required to lay a concrete border around a circular pond. 2. **Given data:** - Diameter of pond = 7 m - Width of surrounding path = 2 m - Depth of concrete = 50 mm = 0.05 m (converted to meters) - 1 cubic meter of concrete requires 106 bags of 20 kg each 3. **Find the volume of concrete needed:** The concrete border forms a ring around the pond. Calculate the area of the outer circle (pond + path) and subtract the area of the pond to get the area of the path. - Radius of pond $r_1 = \frac{7}{2} = 3.5$ m - Radius of outer circle $r_2 = r_1 + 2 = 3.5 + 2 = 5.5$ m Area of pond: $$A_1 = \pi r_1^2 = \pi (3.5)^2 = 12.25\pi$$ Area of outer circle: $$A_2 = \pi r_2^2 = \pi (5.5)^2 = 30.25\pi$$ Area of path: $$A = A_2 - A_1 = 30.25\pi - 12.25\pi = 18\pi$$ 4. **Calculate volume of concrete:** Volume = Area of path $\times$ depth $$V = 18\pi \times 0.05 = 0.9\pi \approx 2.8274 \text{ m}^3$$ 5. **Calculate number of bags:** Each cubic meter requires 106 bags, so: $$\text{Number of bags} = 0.9\pi \times 106 \approx 2.8274 \times 106 = 299.7$$ 6. **Round to nearest whole number:** $$\boxed{300}$$ bags **Final answer:** You need approximately 300 bags of 20 kg premixed concrete.