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?

16 minutes. The 10-minute person and the 7-minute person start, and when the 7-minute person finishes, they relay their flashlight to the 2-minute person, who crosses, then relays the flashlight to the 1-minute person, who crosses, and finishes at the same time as the 10-minute person. Then the 1-minute person takes both flashlights back, gives one to the 5-minute person, and they both cross.

So, full list:

0:00 - 10, 7 leave
7:00 - 7 reaches other side, relays flashlight, 2 leaves
9:00 - 2 reaches other side, relays flashlight, 1 leaves
10:00 - 1 and 10 reach other side, 1 takes both flashlights, leaves
11:00 - 1 reaches 5, both depart
16:00 - 1 and 5 arrive

Posted by Cougar Draven on 2005-06-25 22:50:40