A ring has an outer diameter of 40 cm and a width of 10 cm, what is the radius of its inner circle in cm
一个环形，外圆直径是40厘米，环的宽度是10厘米，它的内圆半径是多少厘米
<gadget id="calculator">40 / 2</gadget>
<output>20</output>

<gadget id="calculator">20 - 10</gadget>
<output>10</output>

<result>10</result>