(4/7) Add 12 to the numerator, the size of the fraction remains the same, how much should be added to the denominator
(4/7)分子加上12，分数的大小不变，分母应该加上多少
<gadget id="calculator">12 + 4</gadget>
<output>16</output>

<gadget id="calculator">16 / 4</gadget>
<output>4</output>

<gadget id="calculator">7 * 4</gadget>
<output>28</output>

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

<result>21</result>