Four fellows are trying to cross a rickety old bridge in the dead of night. The bridge will not hold more than two of them at a time. They have one lamp and it is too dangerous to try to cross without a lamp. The men travel at different speeds: the first one can cross in one minute, the second in two minutes, the third in five minutes and the fourth in ten minutes. Naturally, any two of them can only cross as fast as the slowest of the pair. What is the shortest possible time in which all four can cross?

The answer is here.



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