Add 9 to the numerator of (3/7), to keep the ratio unchanged, how much should be added to the latter term
把(3/7)的分子加上9，要使比值不变，后项应加上多少
<gadget id="calculator">7 * 4</gadget>
<output>28</output>

<gadget id="calculator">28 - 7</gadget>
<output>21</output>

<result>21</result>