If 15 is added to the denominator of the fraction (4/5), how much should be added to the numerator to keep the size of the fraction unchanged.
若分数(4/5)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">16 - 4</gadget>
<output>12</output>

<result>12</result>