U2 has a concert that starts in 17 minutes and they must all cross a bridge to get there. All four men begin on the same side of the bridge. You must help them across to the other side. It is night. There is one flashlight. A maximum of two people can cross at one time. Any party who crosses, either 1 or 2 people, must have the flashlight with them.
The flashlight must be walked back and forth. It cannot be thrown and other tricks like that are not needed to solve the problem. The solution is simply a matter of allocating resources in a certain order. Each band member walks at a different speed. A pair must walk together at the rate of the slower man's pace:
Bono: 1 minute to cross Edge: 2 minutes to cross Adam: 5 minutes to cross Larry: 10 minutes to cross
For example: if Bono and Larry walk across first, 10 minutes have elapsed when they get to the other side of the bridge. If Larry then returns with the flashlight, a total of 20 minutes have passed and you have failed the mission.
[toggle title="What's the answer?"]You get to figure this one out on your own. If you're having a hard time, here's a hint: There is a valid answer that doesn't require tricks like throwing the flashlight or shining it backwards or having some other means of moving the flashlight.
There's an assumption people often make that keeps you from solving this. Two members cross the bridge each time, but neither one of the two the crossed need to return. Think about how that would be possible. If you're still stuck, use objects to simulate their movement. Use whatever you have laying around - pens, paper, erasers - and move them back and forth. Good luck![/toggle]