Four people must cross a rickety bridge at night.
Unfortunately, they have only one torch and the bridge is too dangerous to cross without one.
The bridge can only hold two of them at a time.
The four people cross at different speeds: Times for each person: 1 minute, 2 minutes, 7 minutes and 10 minutes.
What is the shortest time in which they can all cross?
1 and 2 minute-person cross first
2 minute-person comes back
7 and 10 minute-person go across the bridge
1 minute-person comes back
Finally, 1 and 2 minute-person cross the bridge (voilà)
Total time (shortest) = 2 + 2 + 10 + 1 + 2 = 17 mins. 🙂