If 20 is added to the denominator of the fraction (4/5), what is the numerator to keep the fraction constant?
若分数(4/5)的分母加上20，要使分数的大小不变，分子应多少．
<gadget id="calculator">5 + 20</gadget>
<output>25</output>

<gadget id="calculator">25 / 5</gadget>
<output>5</output>

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

<gadget id="calculator">20 - 4</gadget>
<output>16</output>

<result>16</result>