Add 15 to the numerator of (5/6) to keep the size of the fraction unchanged, how much should be added to the denominator.
(5/6)的分子加上15要使分数的大小不变，分母应加上多少．
<gadget id="calculator">5 + 15</gadget>
<output>20</output>

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

<gadget id="calculator">6 * 4</gadget>
<output>24</output>

<gadget id="calculator">24 - 6</gadget>
<output>18</output>

<result>18</result>