An old chestnut
goes like this:
Four people want to cross a rickety old bridge at night. The bridge can only safely hold two people at one time, and can only be crossed safely with a flashlight. There is only one flashlight (to be shared between two people crossing together), so somebody will have to take the flashlight back after each trip.
Each person moves at a different speed. One person can cross the bridge in one minute, another in two minutes, another in five minutes, and the last in ten minutes.
When two people cross together, they move at the slower person's speed.
How fast can the four people all get across the bridge?