1. **State the problem:** We need to find the diameter of a circle given its circumference is 400 meters. 2. **Formula used:** The circumference $C$ of a circle is related to its diameter $d$ by the formula: $$ C = \pi d $$ 3. **Important rule:** To find the diameter, rearrange the formula to solve for $d$: $$ d = \frac{C}{\pi} $$ 4. **Substitute the given value:** $$ d = \frac{400}{\pi} $$ 5. **Calculate the diameter:** Using $\pi \approx 3.1416$, $$ d \approx \frac{400}{3.1416} \approx 127.32 $$ meters. **Final answer:** The diameter of the circle is approximately 127.32 meters.