In 3:8, if 12 is added to the latter term, how much should be added to the former term to keep the ratio constant.
在3：8中，如果后项加上12，要使比值不变，前项应该加上多少．
<gadget id="calculator">3 * 2.5</gadget>
<output>7.5</output>

<gadget id="calculator">7.5 - 3</gadget>
<output>4.5</output>

<result>4.5</result>