Four people need to cross a dark bridge at night with only one flashlight. The bridge holds at most two people at a time, and any crossing requires the flashlight. Person A takes 1 minute to cross, B takes 2 minutes, C takes 7 minutes, and D takes 10 minutes. Pairs walk at the speed of the slower person. What is the minimum time needed for all four to cross?
17 minutes
First, A and B cross together (2 min), and A returns with the flashlight (1 min). Then C and D cross together (10 min), and B returns with the flashlight (2 min). Finally, A and B cross together (2 min). Total time: 2 + 1 + 10 + 2 + 2 = 17 minutes.
About this riddle
Logic riddles give you a few facts and ask what must be true. List what you know, cross out what cannot be, and see what is left.
Difficulty: Hard. More like this: Logic riddles with answers or try Einstein's Logic Grid for a different kind of puzzle.
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