Adam, Ben and Carol are on one side of a bridge in the middle of the night. The bridge can support the weight of only two people at a time. They have one lantern. Because there are holes in the bridge, people crossing the bridge need to be able to see; which is why they brought a lantern. Adam can cross the bridge as fast as or faster than Ben; who can cross the bridge as fast as or faster than Carol. The symbol AB represents Adam and Ben crossing the bridge together. What is the fastest way for everyone to cross the bridge. The answer is AB A AC. The total amount of time it took to get everyone across is A+B+C. That was just an easy version so you understand the problem. Now for the real problems:
Adam, Ben, Carol and Dave want to cross the bridge. Carol can cross the bridge as fast as or faster than Dave. What is the fasted way for everyone to cross the bridge?
The underlined crossings can be switched with equivalent results.
What if Elisabeth want to cross with them?
Ask this question again and again, each time adding a person.
What if an group of n people want to cross?
Note: I got this problem form a TED talk video on Youtube.[/spoiler]