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July 2004

Brainteaser

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

The group, 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 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.

Thanks to the folks at www.grand-illusions.com for cataloging this brainteaser.

want some clues

OK - a few clues.

2 people cross
1 goes back
2 people cross
1 goes back
2 people cross

There are no tricks of people being carried on someone else's back, etc.

OK, which people cross at which time still has to be worked out. There's only 17 mins.

Larry takes 10 mins, Adam 5. You can't allow them to cross separately, since that would be 15 mins, and you'll never solve the puzzle. So they cross together. Which takes 10 mins. You can't allow either of them to make the return trip with the flashlight either, can you? Since they would still have to get back across again. That would take too long.

Have I given you enough clues?

 

just tell me the answer

OK - Here is the answer

  1. Bono and Edge cross the bridge - 2 minutes
  2. Bono returns with the light - 1 minute
  3. Larry and Adam cross - 10 minutes
  4. Edge returns with the light - 2 minutes
  5. Bono and Edge cross the bridge - 2 minutes.

Total 17 minutes!!

The second solution is trivial, and involves swapping steps 2 and 4.

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