Add 14 to the denominator of (3/7), how much should be added to the numerator to keep the size of the fraction unchanged.
(3/7)的分母加上14，要使分数的大小不变，分子应加上多少．
<gadget id="calculator">7 + 14</gadget>
<output>21</output>

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

<gadget id="calculator">3 * 3</gadget>
<output>9</output>

<gadget id="calculator">9 - 3</gadget>
<output>6</output>

<result>6</result>