Four People, One Flashlight
The problem
Four people must cross a bridge at night.
- The bridge holds at most 2 people at a time.
- A single flashlight must travel with whoever is crossing.
- Their crossing times are 1, 2, 5, 10 minutes. Two crossing together move at the slower person's pace.
Get everyone across in 17 minutes. Is it possible?
Tempting (but wrong)
"Use the fastest person as escort." Person 1 walks each slow person across, then runs back:
| Step | Time |
|---|---|
| 1 + 2 cross | 2 |
| 1 returns | 1 |
| 1 + 5 cross | 5 |
| 1 returns | 1 |
| 1 + 10 cross | 10 |
| Total | 19 |
Two minutes over. Many people stop here and decide 17 is impossible.