The ratio of a ratio is (2/7), its preceding term is added by 4, what should be the latter term to keep the ratio constant.
一个比的比值是(2/7)，它的前项加4，要使比值不变后项应为多少．
<gadget id="calculator">2 + 4</gadget>
<output>6</output>

<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<gadget id="calculator">7 * 3</gadget>
<output>21</output>

<result>21</result>