The first a
The second b
The third c
It can be known that no matter how the sequence changes, the waiting time of the third one remains unchanged, even if 10+ 12+8=30 minutes. Then we only need to consider the shortest time of the first and second, the first is 8 minutes, the second is 8+ 10 minutes, and the total time is 8+ 18+30=56 minutes.