A man have three sisters and he gone to her there sisters one by one and her entry or out gate charge rs 5rupie. And her sister in pocket see rupies to double in so how many rupies go to come

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?