Four travelers arrive at a narrow suspension bridge at night holding only one flickering torch. The bridge can safely support at most two people at a time. Their individual crossing times are 1, 2, 5, and 10 minutes. When two cross together, they must move at the speed of the slower person. What is the absolute minimum total time required for all four to reach the other side?
17 minutes
1 and 2 cross (2 min). 1 returns with the torch (1 min). 5 and 10 cross (10 min). 2 returns with the torch (2 min). Finally, 1 and 2 cross together (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: Medium. More like this: Logic riddles with answers or try Einstein's Logic Grid for a different kind of puzzle.
More logic riddles
Two fathers and two sons went on a fishing trip. Each person caught exactly one fish, yet they brought home only three fish in total. How is this possible?
A 3x3 magic square uses the digits 1 through 9 exactly once, such that every row, column, and main diagonal sums to 15. The center square contains 5. If the top-left square contains 1, what number must be in the bottom-right square?
Forward I am heavy, but backward I am not. What am I?
The leprechaun is bigger than the gold coin, and the rainbow is bigger than the leprechaun. Which one is the middle size?
A farmer has four fields (A, B, C, D). Field A produces double the corn of Field B. Field C produces the same amount of corn as Field B. Field D produces no corn at all. Which field produces the least amount of corn?
A vault requires a four-digit numerical code. The first digit is double the last digit. The second digit is three less than the first digit. The third digit is the sum of the first and second digits. If the last digit is 2, what is the code?