Add 12 to the former term of 4:7, to keep the ratio constant, how much should be added to the latter term
把4：7的前项加上12，要使比值不变，后项应加上多少
<gadget id="calculator">4 + 12</gadget>
<output>16</output>

<gadget id="calculator">16 / 4</gadget>
<output>4</output>

<gadget id="calculator">4 * 7</gadget>
<output>28</output>

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

<result>21</result>