Circumference of separate circle So I have been out of Algebra for a while now. I am trying to help my wife prep for an entrance exam and we ran across this in the practice test:
A concrete walkway 2 feet wide surrounds a circular pool of radius 5 feet.
Find the area of the concrete walkway. 

I know that the area of the pool is $153.9384$. ($A = \pi r^2$.)
How do you find the outer circle's square footage though based on that?
No answer please; just the formula. This is a practice test and we want to use the formula not the answer.

In this kind of problem, there is an exterior larger shape enclosing an interior smaller shape. Taking the difference between the enclosed areas of the two will give you the area of the border. 
So subtract the areas of the "small circle" (just the pool) from the "large circle" (which is basically circle formed by the outer border of the walkway).
This method generalises to any regular shape - e.g. rectangular borders surrounding another rectangle and even irregular shapes where you basically have to be given both areas to work out the area of the border.
