Bridge's riddle [duplicate]

You're in a group of people going for an internship in a remote mountain lab. By mistake you pulled a lever with a skull just out of curiosity and it released some zombies. So you and your group had to run.

The janitor, the lab assistant, the professor and you escaped to a bridge near by. However, the bridge could only take the weight of two persons at a time, otherwise it would collapse.

You know that you take 1 minute to crossover the bridge, as for the assistant takes 2 minutes, the janitor takes 5 minutes and the professor takes a whole 10 minutes.

Its dark and you cannot see more than a small range of space provided by your torch.
Those who are without the torch remain quiet and still.


The zombies are coming in about 17 minutes.

Now the puzzle is to find a solution so that you can escape from the zombies and collapse the bridge before they arrive there.

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• This seems rather trivial - your group crosses two at a time, then the zombies collapse the bridge from their own weight. Am I missing something? May 23, 2016 at 16:03
• Are you answering your own question? May 23, 2016 at 16:05
• @Lordofdark see my answer. I believe that's the difference between this question and the one you linked to.
– kuhl
May 23, 2016 at 17:47
• @kuhl look at the video linked at the end, I think it is the exact same thing May 23, 2016 at 17:59
• @Lordofdark you're right hadn't watched the video. Based on the text of the question (ignoring the video) its not a dupe. Because the lantern (lifeboat) isnt mentioned in the question itself. So I'm assuming the text of the question was just a poor summary of the video? That's disappointing...
– kuhl
May 23, 2016 at 18:05

There's nothing in the question to specify the width of the bridge, so I'm assuming that it's possible to pass someone that is ahead of you on the bridge.

Have the Professor and Janitor start crossing the bridge first. Start time + 0 minutes.

Janitor gets to the other side, Assistant starts crossing the bridge. Start time + 5 minutes

Assistant gets to the other side, you begin crossing the bridge. Start time + 7 minutes

You get to the other side. Start time + 8 minutes

Professor gets to the other side Start time + 10 minutes

It will take 10 minutes total for the group to cross the bridge, leaving 7 minutes to collapse the bridge before the zombies arrive.

• even if you disallow overtaking, or standing beside another it's still possible to cross in 13 minutes, T=0 janitor starts, t=1 prof starts , t=5 janitor finishes, t=10 assistant starts , t= 11 professor finishes, t=12 assistant finishes and you start, T=13 everone has crossed. May 24, 2016 at 10:08
• @Jasen not if you watch the video unfortunately. The op forgot to mention that there is one lamp and a lamp is required to cross.
– kuhl
May 24, 2016 at 10:39

Let the Professor cross the bridge first. Time Elapsed = 10 minutes

Then the Janitor, Time Elapsed = 10+5= 15 minutes

Lastly, you and the assistant cross the bridge together. You'll cross it first as you'll only take one minute.

Time Elapsed = 15+1= 16 minutes

The exact moment the assistant reaches the other end of the bridge, the zombies reach the bridge and the bridge collapse.