Add 15 to the denominator of (3/5), how much should be added to the numerator to keep the size of the fraction unchanged.
把(3/5)的分母加上15，要使分数的大小不变，分子应加上多少．
<gadget id="calculator">3 * 4</gadget>
<output>12</output>

<gadget id="calculator">12 - 3</gadget>
<output>9</output>

<result>9</result>