Four Siblings Bridge Crossing Puzzle | MindYourLogic Bridge Crossing Puzzle

In this challenging puzzle, four siblings must cross a narrow bridge at night using a single lantern. Each sibling moves at a different speed, and the lantern must be carried back and forth to ensure that everyone crosses safely. The crossing times for each sibling are: Sibling A takes 1 minute, Sibling B takes 4 minutes, Sibling C takes 6 minutes, and Sibling D takes 9 minutes. The goal is to determine the minimum total time required for all four siblings to cross the bridge, which is optimally 20 minutes. This puzzle requires careful planning and strategic movement to achieve the minimal crossing time while managing the lantern efficiently.

Steps:

First Crossing:

Sibling A (1 minute) and Sibling B (4 minutes) cross the bridge together.
Time Taken: 4 minutes (time of the slower sibling, Sibling B).
Lantern Return:

Sibling A (1 minute) returns with the lantern.
Time Taken: 1 minute.
Total Time So Far: 5 minutes.

Second Crossing:

Sibling C (6 minutes) and Sibling D (9 minutes) cross the bridge together.
Time Taken: 9 minutes (time of the slower sibling, Sibling D).
Total Time So Far: 14 minutes.

Lantern Return:

Sibling B (4 minutes) returns with the lantern.
Time Taken: 4 minutes.
Total Time So Far: 18 minutes.

Final Crossing:

Sibling A (1 minute) and Sibling B (4 minutes) cross the bridge together again.
Time Taken: 4 minutes (time of the slower sibling, Sibling B).
Total Time So Far: 22 minutes.

Conclusion:

By following these carefully planned steps, all four siblings will successfully cross the narrow bridge in a total of 22 minutes. This strategy involves pairing the fastest siblings to handle the lantern efficiently and managing return trips to minimize the overall crossing time. Despite the challenge, optimizing each step helps achieve the goal while meeting the constraints of different crossing speeds and the necessity of the lantern.