In 2:5, add 4 to the former term of the ratio, how much should be added to the latter term to keep the ratio constant.
在2：5中，把比的前项加上4，要使比值不变，比的后项应加上多少．
<gadget id="calculator">2 + 4</gadget>
<output>6</output>

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

<gadget id="calculator">5 * 3</gadget>
<output>15</output>

<gadget id="calculator">15 - 5</gadget>
<output>10</output>

<result>10</result>