Add 12 to the numerator of (3/5), how much should be added to the denominator to keep the size of the fraction unchanged.
(3/5)的分子加上12，要使分数的大小不变，分母应该加上多少．
<gadget id="calculator">12 / 3</gadget>
<output>4</output>

<gadget id="calculator">4 * 3</gadget>
<output>12</output>

<gadget id="calculator">4 - 3</gadget>
<output>1</output>

<gadget id="calculator">3 + 12 + 1 + 4</gadget>
<output>20</output>

<result>20</result>