Lucky key is 8


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

The average is 5.05