Logical Puzzles
Hard
5 min
Four people need to cross a bridge at night. They have one torch. The bridge can support at most two people at a time. Their crossing times are 1, 2, 7 and 10 minutes. When two people cross together, they move at the slower person's speed. What is the minimum total time needed for everyone to cross?
Hint
Use the two fastest people strategically.
Answer
17 minutes.
Explanation
The optimal strategy is to use the two fastest people to transport the torch while the two slowest cross together.
Step-by-Step Solution
1. 1 and 2 cross = 2 minutes.
2. 1 returns = 1 minute.
3. 7 and 10 cross = 10 minutes.
4. 2 returns = 2 minutes.
5. 1 and 2 cross = 2 minutes.
Total = 2 + 1 + 10 + 2 + 2 = 17 minutes.
2. 1 returns = 1 minute.
3. 7 and 10 cross = 10 minutes.
4. 2 returns = 2 minutes.
5. 1 and 2 cross = 2 minutes.
Total = 2 + 1 + 10 + 2 + 2 = 17 minutes.