The Midnight Bridge
Four people come to a river in the night. There is a narrow bridge, but it can only hold two people at a time. They have one torch and, because it's night, the torch has to be used when crossing the bridge. Person A can cross the bridge in one minute, B in two minutes, C in five minutes, and D in ten minutes. When two people cross the bridge together, they must move at the slower person's pace. The question is, how much time do they need?