If the ratio is 3:7, if the former item is increased by 12, how much must be added to the latter item to keep the ratio unchanged.
比3：7如果前项增加12，要使比值不变就得在后项增加多少．
<gadget id="calculator">7 * 5</gadget>
<output>35</output>

<gadget id="calculator">35 - 7</gadget>
<output>28</output>

<result>28</result>