Test Your Intelligence With This Classic Bridge-Crossing Riddle

Some riddles are more than just a bit of fun — they're also an effective intelligence barometer. The following brain-teaser from TED-Ed will test both your maths knowledge and ability to think outside the box. It starts with four people who need to get to the other side of a bridge, each with their own walking speeds and quickly gets complicated...

Got your pen and paper ready? Here is the setup:

Four people need to get to the other side of a bridge in 17 minutes before it collapses. Person A can get across in one minute, Person B in two minutes, Person C in five, and Person D, who presumably has no legs, takes ten.   The bridge can only support two of them for each trip (the person with the longer time is counted when travelling in a pair). To complicate matters further, it’s dark and the party only has one torch — so a person who has crossed the bridge has to come back for a return trip in order to lead another person across. How do you get everybody across in 17 minutes?

The below video contains the full solution. Let us know how you went in the comments!

[Via Gizmodo]


Comments

    Hmm, my solution was somewhat different
    Speedy and the Professor cross the bridge, with Speedy carrying the Professor and the Professor carrying the lamp. The rest of it was as easy as cheesecake once Prof was over the other side.
    Yeah, I value practicality over the laws of mathmatics

      Agree - my thought was carry the damn professor. The rules don't exclude it.

        The bridge can only support two of them for each trip (the person with the longer time is counted when travelling in a pair).

        How is one person carrying another NOT travelling in a pair?

        Last edited 14/09/15 11:48 am

          They are travelling as a pair. They are the only two on the bridge.

    I got a slightly different way. Person A and B cross and then Person B comes back (13mins left). Person C and D cross leaving 3 mins. Then Person A runs back leaving 2 minutes to spare and Person A and B cross together taking up the final 2 minutes. 17-2-2-10-1-2=0

    its a bridge...you walk in a straight line with railings, why do you need a torch?

    Just have the first person C and D walk across with the torch, and A and B (who would take 2 mins to walk) can go at half speed for all I care...(hell, they can crawl at 1/3 speed and still have 1 minute spare).

    The normal solutions works as well ^_^...hasn't this been done before, with animals crossing a river or something...

    Last edited 13/09/15 6:44 am

      Only two people can be on the bridge at a time.

        There will only be 2 people on the bridge at one time. C and D crosses, then uses the light (maybe a quick morse code signal) to signal A and B to start crossing.

        Actually, thinking more, A and B should cross first, then signal back that the other side is safe (assuming the video's scenario of a zombie apocalypse). Then, in a worse case scenario, they can get back before the 17 minutes is up. Though I guess then they'd be stuck with zombies on both sides...

      This puzzle is designed to test your mathematics skills.
      I think it's clear how you fared in this 'intelligence test'.

        The following brain-teaser from TED-Ed will test both your maths knowledge and ability to think outside the box.

        The drive for laziness creates innovation. This was a problem solving test. All you need to do is achieve an outcome based on the rules. It's up to the rule maker to set these rules.

        I didn't say I didn't like the mathematical solution...I just thought in reality (even hypothetically), there'd be a whole lot other factors involved, so the simplest solution is often better than the elegant mathematically sound one.

        For example, will the bridge collapse via catastrophic failure, or will it degrade over time and 17 minute be the final point of collapse? (in which case, walking across in the last 2 minutes would be difficult). Will asking one person to cross multiple times affect his/her crossing times?

        The mathematical solution that everyone put forth is perfect...I just thought it'd be fun to extend the exercise beyond the numbers...

    id have the fast guy piggyback the slow guys.
    job done. with time to spare

    Unless the zombies get their heads cut off? Won't they just get back up from wherever they fall too, keep on moving and slowly kill the whole world? So isn't getting across the bridge a pointless endeavour? If they were breeding zombies in a lab, surely they would have some sort of security team, or weapons?

    Surely they have to neutralise the threat for mankind!

    Last edited 14/09/15 11:46 am

      nah - the bridge takes them to a magic helicopter that for some reason couldn't pick them up from the other side of the bridge.

      The security team all got caught by the zombies for trying to break the rules and piggyback people across the bridge or cross the bridge without a torch.

      As for the weapons, all the weapons were stolen by a nasty trickster who put them inside the magic helicopter.

      It's all in the video, but you really have to understand the subtext

    You = 1min
    Assistant = 2min
    Janitor=5min
    Professor=10min

    You + Assistant (2)
    You go back with light (1)
    Janitor + Professor(10)
    Assistant goes back with light (2)
    You + Assistant (2)

    2+1+10+2+2 = 17

    Boom!

    Last edited 14/09/15 1:37 pm

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