In the ratio formula 7:2=21:6, if 6 is added to the latter term of the first ratio, then how much must be added to the latter term of the second ratio before the equation is established.
在比例式7：2=21：6中，如果第一个比的后项加上6，那么第二个比的后项要加上多少等式才成立．
<gadget id="calculator">24 - 6</gadget>
<output>18</output>

<result>18</result>