Add 21 to the latter term of 2:7, to keep the ratio constant, how much should the former term be.
把2：7的后项加上21，要使比值不变，前项应多少．
<gadget id="calculator">7 + 21</gadget>
<output>28</output>

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

<gadget id="calculator">2 * 4</gadget>
<output>8</output>

<gadget id="calculator">8 - 2</gadget>
<output>6</output>

<result>6</result>