2: The former term of 7 is multiplied by 4, to keep the ratio unchanged, how much should be added to its latter term
2：7的前项乘4，要使比值不变，它的后项应加上多少
<gadget id="calculator">7 * 4</gadget>
<output>28</output>

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

<result>21</result>