Lucky key is 8


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

The average is 5.15