Four people need to cross a rickety rope bridge at night. They have one flashlight. The bridge can hold at most two people at a time. Their crossing times are 1, 2, 5, and 10 minutes. When two cross, they walk at the speed of the slower person. What is the minimum time required for all four to cross?
17 minutes
1 and 2 cross (2 min). 1 returns with flashlight (1 min). 5 and 10 cross (10 min). 2 returns with flashlight (2 min). 1 and 2 cross again (2 min). Total: 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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