It is known that the average of 20, A, and 8 is 13, what is A.
已知20、A、8的平均数是13，A是多少．
<gadget id="calculator">13 * 3</gadget>
<output>39</output>

<gadget id="calculator">39 - 20 - 8</gadget>
<output>11</output>

<result>11</result>