Lucky key is 8
5,2,1,3,4,7,6,8,it took 8 times
3,8,it took 2 times
5,2,1,8,it took 4 times
6,4,2,3,1,7,8,it took 7 times
3,1,8,it took 3 times
4,6,1,7,8,it took 5 times
6,3,1,7,5,4,2,8,it took 8 times
8,it took 1 times
3,7,8,it took 3 times
5,6,2,3,1,7,8,it took 7 times
8,it took 1 times
6,5,1,3,8,it took 5 times
8,it took 1 times
6,8,it took 2 times
2,5,8,it took 3 times
5,6,8,it took 3 times
5,3,7,8,it took 4 times
7,3,4,1,5,8,it took 6 times
6,3,4,5,2,1,8,it took 7 times
6,1,5,8,it took 4 times
The average is 4.2