From the nine numbers 1, 2, 3, 4, 5, 6, 7, 8, and 9, randomly select 3 numbers to form a group so that its average number is 6. How many ways are there?
从1、2、3、4、5、6、7、8、9九个数中，任取3个数组成一组，使它的平均数是6，有多少种取法．
<result>7</result>