○+□=21, □+△=25, ○+△=22, ○=how much.
○+□=21，□+△=25，○+△=22，○=多少．
<gadget id="calculator">25 - 22</gadget>
<output>3</output>

<gadget id="calculator">21 + 3</gadget>
<output>24</output>

<gadget id="calculator">24 / 2</gadget>
<output>12</output>

<gadget id="calculator">21 - 12</gadget>
<output>9</output>

<result>9</result>