A concert starts in seventeen minutes and four guys 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 one or two people) must have the flashlight with them. The flashlight must be walked back and forth. It cannot be thrown etc. Each person walks at a different speed. A pair must walk together at the rate of the slower man's pace.

Bandmate	Time to cross
Bono	1 minute
Edge	2 minutes
Adam	5 minutes
Larry	10 minutes

For example, if Bono and Larry walk across first, ten minutes have elapsed when they get to the other side of the bridge. If Larry then returns with the flashlight, a total of twenty minutes have passed and you have failed the mission. Note: there are two known answers to this problem.

This problem has circulated for some time. We first heard of it from Van Ma at Nicholls State University about a year before this submission.

Click here if you need a hint.

Click here for the solution.