Add 20 to the denominator of (7/10), how much should be added to the numerator to keep the size of the fraction unchanged.
(7/10)的分母加上20，要使分数大小不变，分子应加上多少．
<gadget id="calculator">10 + 20</gadget>
<output>30</output>

<gadget id="calculator">30 / 10</gadget>
<output>3</output>

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

<gadget id="calculator">21 - 7</gadget>
<output>14</output>

<result>14</result>