To think about the volume of the washer, we really just have to think about the area of the face of the washer. So area of face is going to be equal to what? Well, it would be the area of the washer, if it wasn't a washer, if it was just a coin, and then subtract out the area of the part that you're cutting out. So the area of the washer, if it didn't have a hole in the middle, would just be pi times the outer radius squared. It would be pi times this radius squared. That we could call the outer radius. And since it's a washer, we need to subtract out the area of this inner circle.