<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>

Four friends a b c and d need to cross the bridge