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Neurobics

Microsoft Meets U2

Russell Dear

I have it on good authority that the following problem is one given by Microsoft to test prospective employees. It is solveable without any tricks and people usually fail in the given time limit.

The rock group U2 have a concert that starts in 17 minutes and the band members must all cross a bridge to get there. All four men begin on the same side of the bridge. You must tell them how to get across in the 17 minute time limit according to certain rules. It is night and there is one torch. A maximum of two people can cross the bridge at any one time and they must have the torch with them when they travel (this means that if two people cross the bridge they must travel together). The torch cannot be thrown.

Each band member walks at a different speed and when a pair travel together they do so at the rate of the slower man. Bono takes one minute to cross, Edge takes two minutes, Adam takes five minutes and Larry takes ten minutes. So, for example, if Larry and Bono travel across the bridge together, then Bono remains and Larry travels back alone, 20 minutes have elapsed.

How do you get them all across the bridge in 17 minutes?

There are no hidden tricks in the problem. It is simply the movement of resources in an appropriate order. Apparently Microsoft expect you to solve the problem in five minutes. It took me six!

Russell Dear is a Mathematician living in Invercargill