In 7:12, add 7 to the former term of the ratio, to keep the ratio unchanged, what should be the latter term of the ratio.
在7：12中，比的前项加上7，要使比值不变，比的后项应多少．
<gadget id="calculator">12 * 2</gadget>
<output>24</output>

<gadget id="calculator">24 - 12</gadget>
<output>12</output>

<result>12</result>