Three missionaries and three cannibals are on one side of the river, wanting to get across.
Unfortunately, the only boat available can hold a maximum of two people. The missionaries, wanting to stay safe, can never be on a side with more cannibals than missionaries (even for a moment!). The boat cannot travel under its own power, so there must be at least one person on board for it to cross.
How can the missionaries get safely across?
Well, you can't ever have one missionary in the boat alone or two
missionaries in the boat alone, because that would mean that the
cannibals have a majority on one side or another.
So a cannibal has to be the one moving the boat singlehandedly, when single-handed operation is called for.
Simplest Solution:
1) Cannibal 3 (who is able to operate the boat) gets in first.
2) Missionary 1 enters second, handcuffs Cannibal 3 into the boat, in such a way that the Cannibal 3 can still operate a boat.
3) Handcuffed cannibal makes 4.5 round trips, delivering first
Missionary 1, then Cannibal 1, then Missionary 2, then Cannibal 2, then
Missionary 3.
4) Missionary 3 unlocks Cannibal 3 and exits the boat.
5) Cannibal 3 exits the boat.
Spoiler
