Can you solve the river crossing riddle? – Lisa Winer


As a wildfire rages through
the grasslands, three lions and three wildebeest
flee for their lives. To escape the inferno, they must cross over to the left bank
of a crocodile-infested river. Fortunately, there happens
to be a raft nearby. It can carry up to two animals at a time, and needs as least one lion
or wildebeest on board to row it across the river. There’s just one problem. If the lions ever outnumber the
wildebeest on either side of the river, even for a moment, their instincts will kick in,
and the results won’t be pretty. That includes the animals in the boat
when it’s on a given side of the river. What’s the fastest way for all six animals
to get across without the lions stopping for dinner? Pause here if you want
to figure it out for yourself. Answer in: 3 Answer in: 2 Answer in: 1 If you feel stuck on a problem like this, try listing all the decisions you can make
at each point, and the consequences each choice
leads to. For instance, there are five options
for who goes across first: one wildebeest, one lion, two wildebeest, two lions, or one of each. If one animal goes alone, it’ll just have to come straight back. And if two wildebeest cross first, the remaining one will immediately
get eaten. So those options are all out. Sending two lions, or one of each animal, can actually both lead to solutions
in the same number of moves. For the sake of time,
we’ll focus on the second one. One of each animal crosses. Now, if the wildebeest stays
and the lion returns, there will be three lions
on the right bank. Bad news for the two remaining wildebeest. So we need to have the lion
stay on the left bank and the wildebeest go back to the right. Now we have the same five options, but with one lion
already on the left bank. If two wildebeest go,
the one that stays will get eaten, and if one of each animal goes, the wildebeest on the raft
will be outnumbered as soon as it reaches the other side. So that’s a dead end, which means that at the third crossing, only the two lions can go. One gets dropped off, leaving two lions on the left bank. The third lion takes the raft back to
the right bank where the wildebeest are waiting. What now? Well, since we’ve got two lions waiting
on the left bank, the only option is for two wildebeest
to cross. Next, there’s no sense in two wildebeest
going back, since that just reverses the last step. And if two lions go back, they’ll outnumber the wildebeest
on the right bank. So one lion and one wildebeest
take the raft back leaving us with one of each animal
on the left bank and two of each on the right. Again, there’s no point in sending
the lion-wildebeest pair back, so the next trip should be either
a pair of lions or a pair of wildebeest. If the lions go, they’d eat the wildebeest
on the left, so they stay, and the two wildebeest cross instead. Now we’re quite close because the
wildebeest are all where they need to be with safety in numbers. All that’s left is for that one lion
to raft back and bring his fellow lions over
one by one. That makes eleven trips total, the smallest number needed
to get everyone across safely. The solution that involves sending both
lions on the first step works similarly, and also takes eleven crossings. The six animals escape unharmed
from the fire just in time and begin their new lives
across the river. Of course, now that the danger’s passed, it remains to be seen how long their
unlikely alliance will last.

100 thoughts on “Can you solve the river crossing riddle? – Lisa Winer

  1. You didnt count the chance the 6 crocidle will sink the raft and eat them all

  2. they are all participating,so why will the lion will eat the buffalos?

  3. No one is going to mention that the lions' faces have no noses or mouths. The nose and mouth are outside of their manes

  4. In reality the lions or wildebeest would just have taken their homies and left the other species to die

  5. then they realise that a fire is on the other wise of the bank as well and they still die

  6. What if you send 2 lions, then 2 wildebeest, then the last two. It’s all equal and I don’t see any problems.

  7. Atleast there wasn't a hidden rule or condition that made solution difficult.
    They didn't bend any rules here!

  8. 1 wildabeast 1 lion and 1 wildabeast 1lion and at least the least remaining two

  9. If the lion is so friendly with the wilderbeast, that they return to bring them, why would they eat them?

  10. Ted: Throw question
    Me: "Alright animals I'll give u guys a minute to disguess"
    "Roar" "Moan" "Roar Roar" "Moan~"
    "For hell's sake I'll save the crocs"
    Lmao~

  11. Nostalgia! Solved a similar puzzle with 3 priests and 3 devils crossing the river 15 years ago..

  12. I have solved it in 9 trips. So the minimum number of trips is 9 not 11. I can challenge that

  13. it's time the idiots that make up riddles and "problems" understood that intelligence is designed to solve things that NEED to be solved . if an intelligent mind looks at a fake nonsensical problem , it will be dismissed accordingly .

  14. wait, for 1. why didn't the fire get factored into this, was it just waiting for them for 11 TRIPS?
    2.why would lions and prey work together in the first place?
    3. HOW IS A WOOD RAFT WITH A LOT OF WEIGHT ON IT SAFE FROM CROCODILES?

  15. Wait…what's wrong with taking a lion & wildebeast pair across without having to return any on the trip back??…

  16. If they're smart enough to use a raft and work together, they're smart enough to not eat

  17. 3:53
    I SWEAR TO GOD IF YOU EAT THEM AFTER ALL THAT WORK I DID I'M LETTING YOU GET EATEN BY THE CROCS

  18. I love how the animals just wave all friendly-like even as they're running for their lives from a fire

  19. No theres a mininum is 4. cause I lion and wilderbeast for 3 times

  20. Humans: can't solve easy riddles
    Animals: know how to operate a raft and work together

  21. Me during the riddle: i can just put two of each three times
    thinks it over
    Me: yea im right
    thinks it again
    Me im still right

    watches answer

    Me: oh yea the boat still has to get back

  22. Also the answer for the second way is bring 2 lion across
    Bring one back
    Bring 2 lion across
    Bring one back
    Bring 2 wildebeest across
    Bring a wildebeest and lion back
    Bring 2 wildebeest
    Bring a lion back
    Bring 2 lions
    Bring a wildebeest back
    Then finally bring wildebeest and lion across

  23. If the wildebeest outnumber the lions, can the wildebeest gang on the lions and neutralize them or something?

  24. My teacher kinda used this but different :/

    only 1 girl and 3 boys :/

    If the crocodile sees the boys then they will die but the girls won’t die

  25. If the Lions and wildebeest are able to cooperate, the why don't the crocodiles let the animals jump across their backs?

  26. I'm pretty sure I solved this one, but I used the double lion method and I don't want to go through the process of all 11 trips on the graphic they gave so I'm just assuming I didn't miss anything.

  27. when you actually get it without help, i mean it took 30 min but at least it happened

  28. My solution: train the crocodiles
    If you don’t know how to watch how to train your dragon
    Crocodiles are pretty much dragons right?

  29. Ted ed : lions feast on wildbeasts once they outnumber them
    Lions : wave to wild beasts !

  30. ….. assuming that
    1. lions would think twice before eating them
    2. the fire won’t already be there by that time
    3. lions and wildebeests can row a raft

  31. Easy way to solve this:
    -The pack of animals is a binary number, with 1 representing a wildebeest and 0 representing a lion.
    -The right bank currently looks like 111000
    -the left bank is empty
    -there can never be a greater number of zeros then ones one either side
    -1st move
    -right bank: 1110
    -left bank: 00
    -2nd move:
    -right bank: 10
    -left bank: 1100
    -3rd move
    -right bank: 1100
    -left bank: 10
    -4th move
    -right bank: 00
    -left bank: 1110
    -5th and final move
    -right bank:
    -left bank: 111000

  32. In crossing 6 why did two animals need to go
    Just send one wildebeest and bring back one Wildebeest then go get lion and bring him to the left

  33. Excuse me,THE FIRE WOULD HAVE SPREAD BY THEN,IF IT DIDN’T SPREAD THEY WOULD NOT HAVE TO CROSS AND WAIT FOR IT TO DIE DOWN

  34. Can’t you do two lions then two wilderness then the remaining ones of each on the same boat

  35. 1 question, how can lions or beasts steer a raft??? And why don't the crocks eat the animals in the raft???????

  36. Solution:
    Crocs become homies with Wildebeest and they carry them over, then they let the lions take the raft. Wildebeest are out of reach, no one dies!

  37. ok so take the 2 lion over in one raft trip then take the 2 wilder beast and then the wilderbeast and lion over Bam im not sure if im wrong correct me tho

  38. My solution :
    ??<
    ?>
    ??<
    ?>
    There is now 1? and 3?(yeah it's a zebra I know) on the bad side of the river, and 2? on the good side.
    ??< (Leaving one lion with the wildbeest)
    ??>
    Now, there is 1? and 1? on the good side, and 2? and 2? on the bad side.
    ??< (Leaving the two lions alone)
    Now, we can't send back a wilbeest, because it's gonna get eaten if we send it alone, and if we send two it's the same thing but in reverse.
    ?>
    ??<
    ?>
    ??<

    We now have all six animald on the good side of the river.
    Number of crossing: 11
    Casualties: 0

  39. Wait. Those lions have two eyes on one side of their heads, so that must mean they have 4 eyes…?

  40. Why would the lions bother to help the wildebeests if they can easily have their dinner and work it out themselves?

  41. I watched it a long long time ago and then it hit me I knew what it was from my mind

  42. مـــين? يــرحب? بـي? يــــضغـــط لايـك?ويــــراســنـي خـــــاص واشـــترك بقـــناتــــي
    واتـــساب**00212.674.056.812*???

  43. Is no one going to acknowledge the fact that wildebeest and lions can operate rafts?

Leave a Reply

Your email address will not be published. Required fields are marked *