Several different four-digit numbers can be formed by using the four numbers 1, 9, 9, and 8. What is the average value of all these four-digit numbers?
用1，9，9，8四个数字可以组成若干个不同的四位数，所有这些四位数的平均值是多少？
<gadget id="calculator">80_172 / 12</gadget>
<output>6_681</output>

<result>6_681</result>