If the denominator of (3/5) is increased by 20, how much should the numerator be increased to keep the size of the fraction constant?
(3/5)的分母增加20，要使分数的大小不变，分子应增加多少．
<gadget id="calculator">3 * 5</gadget>
<output>15</output>

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

<result>12</result>