<P> Variations of the story often indicate a reason why the bridge must be crossed in as short a time . For instance, in Ted - Ed's version of the riddle, the oncoming approach of zombies necessitates a fast escape . </P> <P> An obvious first idea is that the cost of returning the torch to the people waiting to cross is an unavoidable expense which should be minimized . This strategy makes A the torch bearer, shuttling each person across the bridge: </P> <Table> <Tr> <Th> Elapsed Time </Th> <Th> Starting Side </Th> <Th> Action </Th> <Th> Ending Side </Th> </Tr> <Tr> <Td> 0 minutes </Td> <Td> ABCD </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> 2 minutes </Td> <Td> CD </Td> <Td> A and B cross forward, taking 2 minutes </Td> <Td> AB </Td> </Tr> <Tr> <Td> 3 minutes </Td> <Td> A CD </Td> <Td> A returns, taking 1 minute </Td> <Td> B </Td> </Tr> <Tr> <Td> 8 minutes </Td> <Td> D </Td> <Td> A and C cross forward, taking 5 minutes </Td> <Td> ABC </Td> </Tr> <Tr> <Td> 9 minutes </Td> <Td> A D </Td> <Td> A returns, taking 1 minute </Td> <Td> BC </Td> </Tr> <Tr> <Td> 17 minutes </Td> <Td> </Td> <Td> A and D cross forward, taking 8 minutes </Td> <Td> ABCD </Td> </Tr> </Table> <Tr> <Th> Elapsed Time </Th> <Th> Starting Side </Th> <Th> Action </Th> <Th> Ending Side </Th> </Tr>

How to cross the bridge in 17 minutes
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