2: 4 is added to the former term of 7, and how much is added to the latter term, the ratio remains unchanged.
2：7的前项加4，后项加多少，比值不变．
<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>

<gadget id="calculator">21 - 7</gadget>
<output>14</output>

<result>14</result>