On average, Brett is able to read 4 3/4 chapters in his book each week. How many weeks will it take Brett to read 9 1/2 chapters

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?