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Thursday, February 28, 2008

Five Men and Two Bridges

In the puzzle Crossing the Bridge, we met four people who needed to cross a bridge at night. In this puzzle there are five people who have to cross two sequential bridges at night. Like in the earlier 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 only have two flashlights to share between them. A pair of people can share one flashlight, which means there can be one pair of people on each of the two bridges at the same time.

If the short time it takes to get from the first bridge to the second can be ignored, what is the shortest amount of time it will take for all five people to cross both bridges?

1 comment:

Anonymous said...

I'm sure we can do better, but I'll start things off with 28 minutes.

B1 = bridge 1
B2 = bridge 2
F1 = flashlight 1
F2 = flashlight 2

10,7 cross B1 w/ F1 (10 min)
As 10,7 cross B2 w/ F1
- 2, 1 cross B1 w/ F2
- 1 cross B1 w/ F2
- 1,5 cross B1 w/ F2
wait 2 min for 10,7 to finish. (20 min)
1,5 cross B2 w/ F2 (25 min)
1 cross B2 w/ F2 (26 min)
1,2 cross B2 w/ F2 (28 min)