Use A☆B to represent the average of A and B, that is, A☆B=(A+B)/2, then (3☆9)☆4=how much.
用A☆B表示A和B的平均数，即A☆B=(A+B)/2，则(3☆9)☆4=多少．
<gadget id="calculator">3 + 9</gadget>
<output>12</output>

<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<gadget id="calculator">6 + 4</gadget>
<output>10</output>

<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<result>5</result>