Four people, Alan, Beth, Carol and Dave have to cross a rickety bridge at night. Due to the strength of the bridge and the weight of the people only two people can cross at any one time. Furthermore, it would be too dangerous for anyone to try to cross the bridge without a torch, and there is only one torch.
Alan can cross the bridge in one minute, Beth takes two minutes to cross, while Carol needs five minutes to get across, and Dave's traverse time is ten minutes.
How can they all cross the bridge in 17 minutes?
Alan and Beth cross the bridge together (2 minutes)
Beth returns with the torch (4 minutes)
Carol and Dave go across (14 minutes)
Alan returns with the torch (15 minutes)
Alan and Beth once again cross together (17 minutes)
We discovered this puzzle online and believe that it should be credited, but can't find out who - we'll do this if we find out.