Concentric

Concentric means having the same center.
Two circles which have the same center are concentric:

A common problem you see with concentric circles is like this:
What is the area of the blue ring in this diagram?

This is simply the area of the outer circle (

R²) minus the area of the inner circle (

r²).
So, if R = 6 inches and r = 4 inches, then the blue area (called an 'annulus' or 'annular ring') is

6² -

4² =

(36 - 16) = 3.14 x 20 = 62.8 square inches.

Now, here's a problem for you:
What is the area of the ring in this figure?
(measurements are in inches)

Area = _____ - _____ = ______ square inches.