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Edited: 08/09/2010 04:05 PM
Picture: John Rowland
John Rowland
Cross the bridge
Four people have to cross a narrow bridge at night. The bridge can only take a maximum of two people at a time and they must use a torch. They only have one torch.  Individually they can cross in 1, 2, 5 and 10 minutes. If two travel together they must go at the speed of the slowest. How do they all get across in 17 minutes?
Edited: 20/05/2013 05:13 PM
Picture: John Rowland
John Rowland

The 2 minute and the 1 minute person go across (time = 2 min). Then the 1 minute person comes back (cumulative time = 3 min). Then the 5 and 10 minute people go across (cumulative time = 13 min). Then the two minute person goes back (cumulative time = 15 minutes). Finally the 2 and 1 minute people go back across (cumulative time =17 minutes).

At step two, the 2 minute person can go back and then the 1 minute person goes back at step four - but the result is the same (17 minutes).