Sally ordered placemats for her restaurant. The cost was $20 for the first 300 placemats plus $0.10 for each placemat over 300. If the total
cost of the order was $90, how many placemats did Sally order?

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?