This video from Your Teacher is one of a range of math solutions. (which is the measurement from the center to the edge when drawn in a straight line) and also the area of the circle, is the amount of total space within its perimeter. The circumference of this circle is 20 pi. The circumference is 2 pi r. So 20 pi is 2 pi r. Which factors down to show that 10 is the radius (10=r). The area is pi r squared. As we know the radius is 10, we can work out the area - 100 pi.