Multiply the denominator of (5/9) by 5, how much should the numerator be increased to keep the size of the fraction constant.
把(5/9)的分母乘以5，要使分数的大小不变，分子要增加多少．
<gadget id="calculator">5 * 5</gadget>
<output>25</output>

<gadget id="calculator">25 - 5</gadget>
<output>20</output>

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

<result>4</result>