To keep the size of the fraction (5/9) unchanged, after adding 9 to the denominator, how much should be added to the numerator.
要使分数(5/9)的大小不变，分母加上9后，分子应加上多少．
<gadget id="calculator">9 + 9</gadget>
<output>18</output>

<gadget id="calculator">18 / 9</gadget>
<output>2</output>

<gadget id="calculator">5 * 2</gadget>
<output>10</output>

<gadget id="calculator">10 - 5</gadget>
<output>5</output>

<result>5</result>