(2/5)=((2+□)/(5+20)), how much should be filled in □
(2/5)=((2+□)/(5+20))，□中应填多少
<gadget id="calculator">5 + 20</gadget>
<output>25</output>

<gadget id="calculator">25 / 5</gadget>
<output>5</output>

<gadget id="calculator">2 * 5</gadget>
<output>10</output>

<gadget id="calculator">10 - 2</gadget>
<output>8</output>

<result>8</result>