Three missionaries and three cannibals need to cross a river using a boat that carries at most two people. At no point can cannibals outnumber missionaries on either side. How can they all cross safely?
Hints
Start by moving two cannibals, then ferry missionaries while keeping the boat balanced.
Answer
Send two cannibals across, bring one back. Send two missionaries across, bring one missionary and one cannibal back. Send the last two missionaries across, bring one cannibal back, and finally ferry the remaining two cannibals. Every crossing keeps cannibals from outnumbering missionaries.