Four People crossing the river with different speeds

There is a dark tunnel and four people wants to cross the tunnel. In the tunnel, only two people can travel at a time, and there is only one torch light. Without the torch light, they cannot travel through the tunnel. The four people can travel with different speeds. The first person takes 1 hour to travel through the tunnel. Second person takes 2 hours, third person takes 5 hours, and fourth person takes 10 hours. How can they travel in the least amount of time?



Solution:

1 & 2 cross the tunnel

1 returns

5 & 10 cross the tunnel

2 returns

1 & 2 cross the tunnel

Total Time: 17 hours