Lucky key is 8


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

The average is 5