Brainteaser: Four people come to a river at night. There is a narrow bridge, and they have only one flashlight. The bridge can only hold two people at a time, and the group must cross it in the shortest possible time. Each person walks at a different speed: one can cross the bridge in 1 minute, another in 2 minutes, the third in 5 minutes, and the slowest in 10 minutes. When two people cross the bridge together, they must go at the slower person’s pace. What’s the shortest time for all four to cross the bridge?
Answer:
The sequence of crossings goes as follows:
The 1-minute and 2-minute people cross together (2 minutes).
The 1-minute person goes back (1 minute).
The 5-minute and 10-minute people cross together (10 minutes).
The 2-minute person goes back (2 minutes).
The 1-minute and 2-minute people cross together again (2 minutes).
Total time: 2 + 1 + 10 + 2 + 2 = 17 minutes.
