There is a concert that starts in just 17 minutes and all of the band members must all cross a bridge to get there. The four members begin on the same side of the bridge and you must help them to get across to the other side.
Due the age of the bridge, a maximum of two people can cross at one time. To make matters worse, it is night-time and there is only one torch. The torch is always required when crossing the bridge and the torch must be walked back and forth, it cannot be thrown, etc. Each band member walks at a different speed and a pair must walk together at the rate of the slower man:
Alan takes 1 minute to cross
Bill takes 2 minutes to cross
Carl takes 5 minutes to cross
Dave takes 10 minutes to cross
For example, if Alan and Dave walk across first, it takes them 10 minutes to cross. If Alan then returns with the torch, a total of 11 minutes will have passed. There is no trick behind this, it is the simple movement of resources in the appropriate order.