In 3:5, if 15 is added to the latter term, how much should be added to the former to keep the ratio unchanged.
在3：5中，如果后项加上15，要使比值不变，前相应加上多少．
<gadget id="calculator">3 * 4</gadget>
<output>12</output>

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

<result>9</result>