Three cannibals and three missionaries want to cross a river in a canoe which only holds two people.
One person has to bring the boat back (it can't be pushed, etc.).
The trouble is, if there are ever more cannibals than missionaries the cannibals will eat the missionaries.
How can the six get safely across?
Trip 1 - 2 cannibals.
Trip 2 - 1 cannibal comes back.
Trip 3 - 2 cannibals (leaving 3 missionaries on this bank).
Trip 4 - 1 cannibal comes back.
Trip 5 - 2 missionaries - leaves one of each on this side and 2 of each on the other side.
Trip 6 - 1 cannibal and 1 missionary come back (there are now 2 of each on this side and 1 of each across).
Trip 7 - the last 2 missionaries cross, leaving 2 cannibals on this side and three missionaries and one cannibal across.
The missionaries can now walk along while the cannibal goes back to get his mates.