In 2:7, add 6 to the former term of the ratio, how much should be added to the latter term to keep the ratio constant.
在2：7中，把比的前项加上6，要使比值不变，比的后项应加上多少．
<gadget id="calculator">28 - 7</gadget>
<output>21</output>

<result>21</result>