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?
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1 solution
First, A and B cross first, taking 2 minutes. Second, A returns with the torch, taking 1 min. Next, C and D cross forward, taking 10 mins. Then B comes back with the torch taking 2 mins. Lastly, A and B cross the bridge taking 2 mins. So, 2 + 1 + 10 + 2 + 2 = 17