Four people (A, B, C, D) need to cross a narrow bridge at night with one flashlight. A takes 1 minute to cross, B takes 2 minutes, C takes 5 minutes, and D takes 10 minutes. At most two people can cross at once, moving at the pace of the slower person. What is the minimum time needed for all four to cross?
17 minutes.
A and B cross first (2 min). A returns with the flashlight (1 min, total 3). Then C and D cross together (10 min, total 13). B returns with the flashlight (2 min, total 15). Finally, A and B cross together again (2 min, total 17).
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: Medium. More like this: Logic riddles with answers or try Einstein's Logic Grid for a different kind of puzzle.
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