Can you solve the river crossing riddle? – Lisa Winer

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 Replies to “Can you solve the river crossing riddle? – Lisa Winer”

  1. My solution:
    You must have 1 lion constantly driving the other animals
    so it would go in terms of sides
    1W/2W,2L
    1W,1L/2W,1L
    2W,1L/1W, 1L
    3W,1L/1L
    3W,3L/0W, 0L

  2. Plot twist, the lions barbecue the wildebeests.

    /__/
    •w•
    /.🍖. comment a kitty plush doll eating barbecue.

  3. 2 lions go first, then 2 wildebeests then the last lion and wildebeest.
    Edit: wait nvm I forgot they have to have someone go back to get the others across :/

  4. I think there is a defect in this puzzle. I don't know how much male lions care about saving each other's lives in real life, but it is a reasonable stipulation of the puzzle that any lion will try to save the life of any other lion. It's also reasonable to stipulate that any wildebeest will try to save the life of any other wildebeest. But the solutions to this puzzle have intermediate stages where all three lions are on the far bank. Why would they send one of their group back? The only gain to them in doing so is saving the life of a wildebeest, and they can't be stipulated to care about that since they are perfectly willing to eat one or two wildebeests when outnumbering them.

    I could accept the proposition that the only reason the lions would care is "Well, we know we are not going to eat any of those wildebeests today because they have these crossings worked out so that we never outnumber them, but if we save their lives maybe we can eat one of them later". That kind of thing. But ABSENT assigning that thinking to the lions, this puzzle has that hole in it.

  5. You think animals would do this and know all this no they would just run through but good riddle I almost got it

  6. I know a different one!
    Theres a fox, a chicken and seeds. You have a raft and have to get all of them to the other side.
    If you take only the seeds, the fox will eat the chicken, and if you take only the fox, the chicken will eat the seeds, and you can bring two with you, what do you do?
    .
    .
    This one is simpler than the one in the video, you bring the fox and the seeds, and then go back and get the chicken

  7. So, +Ted-Ed Riddles, I just solved it without any help in 9 steps and when I saw solution, I wondered that it says minimum steps are 11. I checked again and mine is correct in all respects and is in 9 steps.

  8. i remember doing this kind of puzzle on poptropica when i was little and i was so stumped for a while and in the middle of the night the solution hit me like a ton of bricks.

  9. Here’s my solution I know it doesn’t make sense for them but here’s how it goes first send out two wildebeest then send out two lions then send out One of each and it’s done

  10. How do animals have the ability to control a raft, how do animals know this complicated solution, and Isn't the fire going to reach them according to the vid it says "as a wildfire rages through the grasslands"

  11. This is my Solution

    Two lions cross the river ,they push the raft too the other side, and run away, two wilderbeasts cross the river, they push the raft too the other side, and run away one wilderbeast and one lion cross the river, and run away

  12. 0:50
    Ted ed ( or Teddy Eddie as I like to call it ) – What's the fastest way for all six animals to get across?
    Me – why does it matter to me what's gonna happen to these animals? Now THATS a riddle to talk about!

  13. 1:50 when the lion gets back, just shove him on the beach and take the two wilder beasts. Then you’re done after you take the lions.

  14. These lions cant control their instincts and you expect them to be able to come up with that strategy? Xd

  15. There are three river crossing riddles that are difficult, 4 couples crossing wherein cross coupling is forbidden: two mothers with twins and a police with favour wherein the children can travel only with their mother or police, and the most complicated one a farmer with two cabbage, two goat, two bulls and one wolf.
    I solved the first two but till now not able to solve the last one

  16. If the lions are sensible enough to not eat the wildebeest while they’re on the raft, why not just…don’t? Just don’t eat them at all? Or better yet, just jump on the crocodiles!
    Come on, guys! Use some common sense!

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

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