Is this a rewording of the situation where there is one torch and the 4 "rabbits" take 1, 2, 5 and 10 minutes each respectively to cross and they can only go 2 at a time and must go at the pace of the slowest of the two.
If it is the answer is 17 minutes
1. The two fastest cross (elapsed time 2 minutes)
2. The fastest comes back with the torch (1 minute) - elapsed time now 3 minutes
3. Hands torch to the two slowest who cross (10 minutes) - elapsed time now 13 minutes
4. Hands torch to the second fastest who is already on the other side and he returns (2 minutes) - elapsed time 15 minutes
5. Two fastest cross (2 minutes) - elapsed time 17 minutes
The secret is to get the two slowest rabbits to cross at the same time!
Now if your puzzle is not a rewording of that one, then I guess it is necessary for you to tell us which one it is!