Solving systems using one substitution

Because of your favorite lemonade it is $3.84 for 3 gallons. write this as an unit rate

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?

When you find the area of a triangle do you use in. or in2