Several different three-digit numbers can be formed by using the three numbers 1, 9, and 8. What is the average of all these three-digit numbers?
用1，9，8三个数字可以组成若干个不同的三位数，所有这些三位数的平均数是多少．
<gadget id="calculator">198 + 189 + 891 + 819 + 981 + 918</gadget>
<output>3_996</output>

<gadget id="calculator">3_996 / 6</gadget>
<output>666</output>

<result>666</result>