After the denominator of (7/15) is tripled, how much should be added to the numerator in order to keep the size of the fraction unchanged.
(7/15)的分母扩大3倍后，为了使分数的大小不变，分子应加上多少．
<gadget id="calculator">7 * 3</gadget>
<output>21</output>

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

<result>14</result>