Expand the denominator of (3/7) by 4 times, how much should the numerator be increased to keep the size of the fraction constant.
把(3/7)的分母扩大4倍，要使分数的大小不变，分子应增加多少．
<gadget id="calculator">3 * 4</gadget>
<output>12</output>

<gadget id="calculator">12 - 3</gadget>
<output>9</output>

<result>9</result>