In the puzzle Four people on a Bridge we met four people who needed to cross a bridge at night. In this puzzle, there are five people who must cross two sequential bridges at night. As in the original puzzle, there are some hindrances:

The bridges can only support two people crossing at a time.

Each person has a different speed in which they can cross: 10 minutes, 7 minutes, 5 minutes, 2 minutes, and 1 minute.

They have only two flashlights to share among them.

What is the shortest amount of time it will take for all five people to cross both bridges?

I get 28 minutes, as follows:

T=0:      1,2,5,7,10,F,F    |=====|    (nobody)    |=====|            (nobody)
T=10:    1,2,5,F              |=====|    7,10,F        |=====|            (nobody)
T=12:    5                      |=====|    1,2,F          |=7,10,F====|   (nobody)
T=13:    5,1,F                 |=====|    2               |==7,10,F===|   (nobody)
T=18:    (nobody)           |=====|    1,2,5,F       |====7,10,F=|   (nobody)
T=20:    (nobody)           |=====|    1,2,5,F       |=====|            7,10,F
T=22:    (nobody)           |=====|    5               |=====|            1,2,7,10,F,F
T=23:    (nobody)           |=====|    1,5,F          |=====|            2,7,10,F,F
T=28:    (nobody)           |=====|    (nobody)    |=====|            1,2,5,7,10,F,F

 

 

Posted by Ken Haley on 2005-06-25 17:03:47