Lucky key is 8


2,7,5,8,it took 4 times
7,3,4,2,6,5,1,8,it took 8 times
3,5,2,4,7,8,it took 6 times
5,3,2,7,4,1,8,it took 7 times
1,6,8,it took 3 times
4,2,8,it took 3 times
6,8,it took 2 times
2,5,6,1,3,4,7,8,it took 8 times
8,it took 1 times
6,5,8,it took 3 times
7,2,4,1,8,it took 5 times
3,6,5,8,it took 4 times
7,6,2,8,it took 4 times
2,3,5,6,1,7,8,it took 7 times
3,4,7,5,8,it took 5 times
7,6,8,it took 3 times
7,1,5,3,8,it took 5 times
4,1,5,2,3,6,7,8,it took 8 times
3,8,it took 2 times
7,1,8,it took 3 times

The average is 4.55