A21000 gallon pool has 9000 gallons of water when a hose is placed in pool and add water at a rate of 1500 gallons per hour. how long does it take to fill

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?