Lucky key is 8


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

The average is 4.5