Given the fact that one ppm contaminant in a 55-gallon drum full of water equates with approximately 4 drops of that contaminant, what fraction of one drop per 55 gallons equates with 0.02 ppm

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?