Lucky key is 8
1,3,5,6,4,7,2,8,it took 8 times
7,6,5,3,2,8,it took 6 times
5,3,6,4,2,7,8,it took 7 times
4,8,it took 2 times
8,it took 1 times
8,it took 1 times
6,1,2,4,7,3,8,it took 7 times
5,7,1,6,3,2,4,8,it took 8 times
8,it took 1 times
7,4,5,3,2,1,6,8,it took 8 times
5,4,8,it took 3 times
7,6,2,3,1,4,5,8,it took 8 times
2,4,1,5,7,8,it took 6 times
6,7,5,3,8,it took 5 times
3,6,1,4,2,5,7,8,it took 8 times
6,2,5,1,8,it took 5 times
3,2,4,8,it took 4 times
3,7,2,6,5,4,1,8,it took 8 times
8,it took 1 times
7,5,2,4,3,1,8,it took 7 times
The average is 5.2