There are four numbers, they are 6, X, 2, 4, their average is 5, what is X.
有四个数，它们分别是6、X、2、4，它们的平均数是5，X是多少．
<gadget id="calculator">5 * 4</gadget>
<output>20</output>

<gadget id="calculator">20 - 6 - 2 - 4</gadget>
<output>8</output>

<result>8</result>