Lucky key is 8
7,8,it took 2 times
3,4,2,7,5,1,8,it took 7 times
1,6,5,2,7,3,4,8,it took 8 times
2,3,5,7,6,4,8,it took 7 times
6,1,4,8,it took 4 times
2,7,5,4,3,1,8,it took 7 times
5,7,2,3,1,8,it took 6 times
6,4,2,7,3,1,5,8,it took 8 times
6,4,8,it took 3 times
5,3,1,2,7,8,it took 6 times
2,7,6,3,4,1,5,8,it took 8 times
3,2,7,8,it took 4 times
5,8,it took 2 times
1,6,3,5,2,8,it took 6 times
4,2,8,it took 3 times
6,8,it took 2 times
7,3,8,it took 3 times
6,3,5,7,2,1,4,8,it took 8 times
5,4,7,1,6,3,8,it took 7 times
1,6,2,8,it took 4 times
The average is 5.25