U2 Bridge Brainteaser

Allegedly, this is one of the questions for potential Microsoft employees.

I must warn you, you can really get caught up trying to solve this problem. Reportedly, one guy solved it by writing a C program, although that took him 37 minutes to develop (compiled and ran on the 1st try though). Another guy solved it in three minutes. A group of 50, at Motorola, couldn't figure it out at all. See how long it takes you.


"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, etc. 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.

Notes: There is no trick behind this. It is the simple movement of resources in the appropriate order. There are two known answers to this problem.

This is based on a question Microsoft gives to all prospective employees. Microsoft expects you to answer this question in under 5 minutes!

Good Luck!


Created: 16-July-1998

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