If a set of data 2, 5, 7, the average of a is 7, what is the value of a
如果一组数据2，5，7，a的平均数是7，则a的值是多少
<gadget id="calculator">7 * 4</gadget>
<output>28</output>

<gadget id="calculator">28 - 2 - 5 - 7</gadget>
<output>14</output>

<result>14</result>