Clare was solving an equation but when she checked her answer she saw her solution was incorrect. she knows she made a mistake but she can't find it where is clare's mistake

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}400? m. what is the diameter dd of the lake?