Please explain how to do this

How do you find the area of a triangle

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?

If you measured the width of a window in inches and then in feet with measurement would you have the greater number of units