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An Eternal Quest for Incremental Improvement

Puzzle of the week - Week #46

There are no tricks - this is a straight forward problem. This is supposed to be one of the questions which potential Microsoft employees are asked.

U2 have 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 that crosses the bridge, either 1 or 2 people, must have the flashlight with them. The flashlight must be carried 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 by the time 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.

There is no trick to this. It is a simple movement of resources in the appropriate order. There are two known answers to this problem. Microsoft expects you to answer this question in under 5 minutes!


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