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, 2, 7, and 10 minutes. What is the absolute minimum time required for all four to cross?
17 minutes
1 and 2 cross (2 min). 1 returns with flashlight (1 min). 7 and 10 cross (10 min). 2 returns with flashlight (2 min). 1 and 2 cross again (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.
More logic riddles
A clock gains 3 minutes every single hour. If the clock is set to the correct time right now at noon, what time will the clock show when the actual correct time is 4:00 PM on the same day?
A clock tower takes 6 seconds to strike 4 o'clock (striking 4 times with equal intervals between strikes). How many seconds will it take to strike 8 o'clock?
The carrot is smaller than the rabbit, and the rabbit is smaller than the basket. Which one is the biggest?
A doctor gives you three pills and tells you to take one pill every half hour. Exactly how long will it take you to finish taking all the pills?
A lily pad on a quiet pond doubles in area every day. If it takes 48 days for the lily pad to completely cover the entire surface of the pond, on which day was the pond exactly half-covered?
Three identical boxes are labeled 'Apples', 'Oranges', and 'Apples and Oranges'. Every single label is guaranteed to be incorrect. You are allowed to pull out just one piece of fruit from a single box of your choice to figure out all the correct labels. Which box do you choose?