Can you solve the river crossing riddle? - Lisa Winer

譯者: Xiang Lei- 審譯者: 瑞文Eleven 林Lim

As a wildfire rages through the grasslands,

草原上野火蔓延

three lions and three wildebeest flee for their lives.

三隻獅子和三隻牛羚逃離家園

To escape the inferno,

為了逃離地獄

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

牠們必須跨越潛伏著 鱷魚的河到左岸

Fortunately, there happens to be a raft nearby.

幸運的是，附近有個木筏

It can carry up to two animals at a time,

它一次可以承載兩隻動物

and needs as least one lion or wildebeest on board

並需要至少一隻獅子或牛羚

to row it across the river.

並行橫跨過河

There's just one problem.

只有一個問題

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

在河的兩邊 如果獅子的數量超過牛羚

even for a moment,

不到一會兒

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

獅子的本能會吃了牛羚 這樣結果不會很 OK

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

在渡河的船上，情況也是如此

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

什麼是能讓這六隻動物最快渡河

without the lions stopping for dinner?

又不會讓獅子吃掉牛羚的方式？

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

如果你想自己找出答案，請暫停

Answer in: 3

3

Answer in: 2

2

Answer in: 1

1

If you feel stuck on a problem like this,

如果你覺得這問題困擾你

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

試著列出你在 每個時間點做出的決定

and the consequences each choice leads to.

和每個選擇導致的後果

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

例如：有五個選項，誰先穿過？

one wildebeest,

一隻牛羚

one lion,

一隻獅子

two wildebeest,

兩隻牛羚

two lions,

兩隻獅子

or one of each.

或兩種動物各一隻

If one animal goes alone,

如果一隻動物獨自走

it'll just have to come straight back.

它只需要直接回來

And if two wildebeest cross first,

如果兩隻牛羚先走

the remaining one will immediately get eaten.

剩下的一隻立即被吃了

So those options are all out.

所以這些錯誤的選項都刪除了

Sending two lions,

運送兩隻獅子

or one of each animal,

或每種動物各一隻

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

這兩種方法最終得到的 運送次數都是一樣的

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

因為時間的緣故，我們將關注第二種

One of each animal crosses.

每種動物各一隻

Now, if the wildebeest stays and the lion returns,

現在，如果留下牛羚 讓獅子回來

there will be three lions on the right bank.

會有三隻獅子在右岸

Bad news for the two remaining wildebeest.

壞消息是兩隻牛羚會被吃

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

所以我們需要獅子留在左岸

and the wildebeest go back to the right.

而牛羚回來右岸

Now we have the same five options,

現在我們有五個相同的選項

but with one lion already on the left bank.

但是有一隻獅子在左岸

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

如果兩隻牛羚過去 留下來的那隻會被吃

and if one of each animal goes,

如果每隻動物各一隻過去

the wildebeest on the raft will be outnumbered

一旦到達另一邊

左岸的獅子數量將超過牛羚

as soon as it reaches the other side.

So that's a dead end,

這是糟糕的選擇

which means that at the third crossing,

這意味著在第三次過河時

only the two lions can go.

只有兩隻獅子可以去

One gets dropped off,

一隻獅子回去

leaving two lions on the left bank.

留下兩隻獅子在左岸

The third lion takes the raft back to the right bank

第三隻獅子帶著木筏回來

where the wildebeest are waiting.

右岸的牛羚正在等待

What now?

現在該怎麼辦？

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

好了，因為我們有兩隻獅子在左岸

the only option is for two wildebeest to cross.

唯一的選擇是兩隻牛羚過去

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

接下來，如果兩隻牛羚 再回去沒有意義

since that just reverses the last step.

只是重複這一步

And if two lions go back,

如果兩隻獅子回來

they'll outnumber the wildebeest on the right bank.

在右岸，牠們的數量會超過牛羚

So one lion and one wildebeest take the raft back

所以一隻獅子和一隻牛羚 把木筏帶回來

leaving us with one of each animal on the left bank

在左岸，留下每種動物各一隻

and two of each on the right.

及各兩隻在右岸

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

再次運送每種動物各一隻，沒有意義

so the next trip should be either a pair of lions

所以下一次過河應該是一對獅子

or a pair of wildebeest.

或一對牛羚

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

如果獅子去，在左岸時 牠們會吃掉牛羚，所以牠們留下來

and the two wildebeest cross instead.

所以兩隻牛羚過去

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

現在我們接進尾聲，因為 牛羚都在牠們需要待的地方

with safety in numbers.

在數量上很安全

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

剩下的就是那一隻獅子帶木筏回來

and bring his fellow lions over one by one.

並帶上另一隻獅子，並逐個帶回

That makes eleven trips total,

這總共十一趟來回

the smallest number needed to get everyone across safely.

以最少的次數 安全地讓每隻動物到達對岸

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

先送兩隻獅子過去的解法與這個相似

and also takes eleven crossings.

也需要來回十一趟

The six animals escape unharmed from the fire just in time

六隻動物不會受傷害 很及時的逃過火災

and begin their new lives across the river.

橫跨過河後，並開始新的生活

Of course, now that the danger's passed,

當然，現在危險已經過去了

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

這個不可能的聯盟能維持多久 將持續受到關注