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As a wildfire rages through[br]the grasslands,

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three lions and three wildebeest[br]flee for their lives.

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To escape the inferno,

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the must cross over to the left bank[br]of a crocodile-infested river.

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Fortunately, there happens[br]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[br]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 every outnumber the[br]wildebeest on either side of the river,

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even for a moment,

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

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That includes the animals in the boat[br]when its on a given side of the river.

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What's the fast way for all six animals[br]to get across

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without the lions stopping for dinner?

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Pause here if you want [br]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[br]at each point,

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and the consequences each choice[br]leads to.

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For instance, there are five options[br]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 will 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[br]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[br]in the same number of moves.

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For the sake of time,[br]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 [br]and the lion returns,

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there will be three lions[br]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[br]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 [br]already on the left bank.

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If two wildebeest go,[br]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[br]will be outnumbered

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as soon as it reached the otherside.

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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[br]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[br]on the left bank,

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the only option is for two wildebeest[br]to cross.

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Next, there's no sense in two wildebeest[br]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[br]on the right bank.

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So one lion and one wildebeest[br]take the raft back

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leaving us with one of each animal[br]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 [br]the lion-wildebeest pair back,

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so the next trip should be either[br]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[br]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[br]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[br]to raft back

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and bring his fellow lions over[br]one by one.

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That makes eleven trips total,

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the smallest number needed[br]to get everyone across safely.

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The solution that involves sending both[br]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[br]from the fire just in time

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and begin their new lives [br]across the river.

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Of course, now that the danger's passed,

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it remains to be see how long their[br]unlikely alliance will last.