The four Patrol Leaders from 1^st Gilwell Scout Troop are out on a night hike. They have to catch a train that leaves in 17 minutes from the other side of the river and there is only one bridge which they all must cross to get to the station. 

All four begin on the same side of the bridge and require your directions to catch the train on time. There is only one torch and the bridge can only support the weight of two people at a time. Any group that crosses (either 1 or 2 people) must carry the torch. The torch must be walked back and forth (it cannot be thrown etc)

Each person walks at a different speed but a pair must walk at the speed of the slower person.

Alice takes 1 minute to cross, Barry takes 2 mins, Colin takes 5 mins, David takes 10 mins

For example If Alice and David walk across first 10 minutes have passed when they get to the other side of the bridge.  If David returns with the torch 20 minutes have passed and they have missed the train (which by the way is running on time and will not wait for late passengers)  

There is no trick behind this. It is the simple movement of people in the appropriate order. There are two known answers and the first Cub Scout to find a correct answer will receive a prize for their skill.