Four people need to cross a rickety rope bridge at night. They have only one flashlight, and the bridge can hold at most two people at a time. Their walking speeds are 1 minute, 2 minutes, 7 minutes, and 10 minutes respectively. When two people cross together, they must walk at the pace of the slower person. What is the minimum total time required for all four to cross?
17 minutes
1. The 1-min and 2-min cross together (2 mins). 2. The 1-min returns with the flashlight (1 min). 3. The 7-min and 10-min cross together (10 mins). 4. The 2-min returns with the flashlight (2 mins). 5. The 1-min and 2-min cross together again (2 mins). 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: Medium. More like this: Logic riddles with answers or try Einstein's Logic Grid for a different kind of puzzle.
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