Add 14 to the former item of 7:13, to keep the ratio unchanged, how much should be added to the latter item.
把7：13的前项加上14，要使比值不变，后项应加上多少．
<gadget id="calculator">13 * 3</gadget>
<output>39</output>

<gadget id="calculator">39 - 13</gadget>
<output>26</output>

<result>26</result>