Four people need to cross a dark bridge at night with only one flashlight. A maximum of two people can cross at a time, moving at the pace of the slower person, and anyone crossing must carry the flashlight. Person A takes 1 minute, B takes 2 minutes, C takes 7 minutes, and D takes 10 minutes. What is the minimum total time required for all four to reach the other side?
17 minutes
First, A and B cross (2 mins). A returns with the flashlight (1 min, total 3). Next, C and D cross together (10 mins, total 13). B returns with the flashlight (2 mins, total 15). Finally, A and B cross together (2 mins, 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: Hard. More like this: Logic riddles with answers or try Einstein's Logic Grid for a different kind of puzzle.
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