Add 15 to the denominator of (4/5), how much should the numerator be multiplied to keep the size of the fraction constant?
将(4/5)的分母加上15，要使分数的大小不变，分子要乘以多少．
<gadget id="calculator">5 + 15</gadget>
<output>20</output>

<gadget id="calculator">20 / 5</gadget>
<output>4</output>

<result>4</result>