Add 14 to the denominator of (6/7), what is the numerator to keep the fraction size constant?
(6/7)的分母加上14，要使分数大小不变，分子应该多少．
<gadget id="calculator">7 + 14</gadget>
<output>21</output>

<gadget id="calculator">21 / 7</gadget>
<output>3</output>

<gadget id="calculator">6 * 3</gadget>
<output>18</output>

<gadget id="calculator">18 - 6</gadget>
<output>12</output>

<result>12</result>