Crossing Bridge Interview Puzzle
Four people are on this side of the bridge. the bridge will be destroyed by a bomb in 17 minutes. everyone has to get across before that. Problem is that it’s dark and so you can’t cross the bridge without a flashlight, and they only have one flashlight. Plus the bridge is only big enough for two people to cross at once. The four people walk at different speeds: A fella is so fast it only takes him 1 minute to cross the bridge, B takes 2 minutes, C 5 minutes, the D it takes 10 minutes to cross the bridge. when two people cross the bridge together (sharing the flashlight),
they both walk at the slower person’s pace. can they all get across before the bridge blows up? Mentioning the speeds once again :
Person A: 1 minute
Person B: 2 minutes
Person C: 5 minutes
Person D:10 minutes
Of course its possible, otherwise it wouldn’t be a very interesting question. The only trick is in realizing that you want to get the two slowest people across together, because otherwise you are wasting too much time. Then once you get them across, how do you not make one of them walk back with the flashlight. Well, you just have one of the fast people already there waiting to sprint the flashlight back across.1. A & B cross. total time: 2 minutes.
C ,D |==========================| A,B(flash light)
2. B comes back. total time: 4 minutes.
3. C & D cross. total time: 14 minutes.
B |==========================| A,C,D(FL)
4. A comes back. total time: 15 minutes.
B,A(FL)|==========================| C ,D
5. A & B cross. total time: 17 minutes.
|========= =============| A,B,C,D
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