You are standing next to a really big circular lake. You want to measure the diameter of the lake, but you don't want to have to swim across with a measuring tape! You decide to walk around the perimeter of the lake and measure its circumference, and find that it's $400\pi\text{ m}$. **What is the diameter $d$ of the lake?** $d=$ [[☃ input-number 1]] $\text{m}$