Add 40 to the denominator of (3/10), how much should be added to the numerator to keep the size of the fraction constant.
(3/10)的分母加上40，要使分数的大小不变，分子应加上多少．
<gadget id="calculator">10 + 40</gadget>
<output>50</output>

<gadget id="calculator">50 / 10</gadget>
<output>5</output>

<gadget id="calculator">3 * 5</gadget>
<output>15</output>

<gadget id="calculator">15 - 3</gadget>
<output>12</output>

<result>12</result>