18. Elijah is making a batch of cookies. He needs 1 cups of butter.
A pound of butter cost $3.50.
A pound of butter contains 2 cups.
What is the cost of butter for the batch of cookies?

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?