In 2:7, if the former term of the ratio is increased by 6, how much should the latter term be increased to keep the ratio constant.
在2：7中，如果比的前项增加6，要使比值不变，后项应增加多少．
<gadget id="calculator">7 * 4</gadget>
<output>28</output>

<gadget id="calculator">28 - 7</gadget>
<output>21</output>

<result>21</result>