WEBVTT

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As a wildfire rages through
the grasslands,

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three lions and three wildebeest
flee for their lives.

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To escape the inferno,

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they must cross over to the left bank
of a crocodile-infested river.

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Fortunately, there happens
to be a raft nearby.

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It can carry up to two animals at a time,

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and needs as least one lion
or wildebeest on board

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to row it across the river.

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There's just one problem.

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If the lions ever outnumber the
wildebeest on either side of the river,

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even for a moment,

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their instincts will kick in,
and the results won't be pretty.

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That includes the animals in the boat
when it's on a given side of the river.

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What's the fastest way for all six animals
to get across

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without the lions stopping for dinner?

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Pause here if you want 
to figure it out for yourself.

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Answer in: 3

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Answer in: 2

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Answer in: 1

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If you feel stuck on a problem like this,

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try listing all the decisions you can make
at each point,

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and the consequences each choice
leads to.

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For instance, there are five options
for who goes across first:

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one wildebeest,

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one lion,

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two wildebeest,

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two lions,

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or one of each.

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If one animal goes alone,

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it'll just have to come straight back.

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And if two wildebeest cross first,

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the remaining one will immediately
get eaten.

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So those options are all out.

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Sending two lions,

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or one of each animal,

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can actually both lead to solutions
in the same number of moves.

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For the sake of time,
we'll focus on the second one.

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One of each animal crosses.

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Now, if the wildebeest stays 
and the lion returns,

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there will be three lions
on the right bank.

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Bad news for the two remaining wildebeest.

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So we need to have the lion
stay on the left bank

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and the wildebeest go back to the right.

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Now we have the same five options,

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but with one lion 
already on the left bank.

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If two wildebeest go,
the one that stays will get eaten,

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and if one of each animal goes,

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the wildebeest on the raft
will be outnumbered

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as soon as it reaches the other side.

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So that's a dead end,

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which means that at the third crossing,

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only the two lions can go.

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One gets dropped off,

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leaving two lions on the left bank.

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The third lion takes the raft back to
the right bank

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where the wildebeest are waiting.

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What now?

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Well, since we've got two lions waiting
on the left bank,

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the only option is for two wildebeest
to cross.

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Next, there's no sense in two wildebeest
going back,

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since that just reverses the last step.

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And if two lions go back,

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they'll outnumber the wildebeest
on the right bank.

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So one lion and one wildebeest
take the raft back

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leaving us with one of each animal
on the left bank

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and two of each on the right.

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Again, there's no point in sending 
the lion-wildebeest pair back,

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so the next trip should be either
a pair of lions

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or a pair of wildebeest.

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If the lions go, they'd eat the wildebeest
on the left, so they stay,

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and the two wildebeest cross instead.

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Now we're quite close because the
wildebeest are all where they need to be

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with safety in numbers.

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All that's left is for that one lion
to raft back

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and bring his fellow lions over
one by one.

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That makes eleven trips total,

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the smallest number needed
to get everyone across safely.

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The solution that involves sending both
lions on the first step works similarly,

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and also takes eleven crossings.

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The six animals escape unharmed
from the fire just in time

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and begin their new lives 
across the river.

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Of course, now that the danger's passed,

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it remains to be seen how long their
unlikely alliance will last.