Andy has an hour to study for french exam. it takes him five minutes to write a practice essay and six minutes to review the vocabulary from a lesson. and he wants to write x essays and review the vocabulary in y lessons.

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?