Lucky key is 8
8,it took 1 times
1,7,4,6,3,5,8,it took 7 times
1,2,6,4,5,7,8,it took 7 times
4,6,8,it took 3 times
1,4,6,7,5,3,8,it took 7 times
5,3,6,4,7,8,it took 6 times
7,8,it took 2 times
5,7,1,2,3,8,it took 6 times
3,6,1,8,it took 4 times
7,8,it took 2 times
3,4,6,2,1,5,8,it took 7 times
2,4,6,3,5,1,7,8,it took 8 times
4,1,8,it took 3 times
2,6,8,it took 3 times
2,6,1,8,it took 4 times
6,3,2,7,8,it took 5 times
5,4,8,it took 3 times
5,8,it took 2 times
6,1,3,2,8,it took 5 times
3,6,2,1,7,5,8,it took 7 times
The average is 4.6