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.

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  1. 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

  2. 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!

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

  5. 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"

  6. 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 .

  7. 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?

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

  9. 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

  10. 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

  11. 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

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

  13. 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.

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

  15. ….. 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

  16. 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

  17. 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


  19. 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!

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