Suppose u = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10} is the universal set, and p = {1, 3, 5, 7, 9}. what is p'?

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?