For a ring, the radius of the inner circle is 50% of the radius of the outer circle, and the area of the ring is equal to the area of the outer circle.
一个圆环，内圆半径是外圆半径的50%，这个圆环的面积是外圆面积的多少．
<gadget id="calculator">75 / 100</gadget>
<output>3/4 = around 0.75</output>

<result>3/4 = around 0.75</result>