A store that sells video games gives a 15% discount for customers who trade in a used game. Alex brought a game
to trade, and bought a game that costs $29.99. How much
will the game cost (before tax)?

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?