# Crossing the bridge



## hiker95 (Nov 23, 2012)

Crossing the bridge

4 people are crossing a bridge at night, and they have 1 lantern between them.

They reach a very, very, deep gorge, spanned by a log bridge.

As with all such puzzles, there are various artificial constraints:  
1. They must cross the gorge, on the bridge, in less than 18 minutes. 
2. Only two people can be on the bridge at the same time or it will not hold them.
3. They need the lantern to cross; they can't cross in the dark. 
4. It's a long bridge and an old-fashioned lantern. You can barely see your feet; you can't see very far ahead or behind on the bridge. 
5. It's OK to be left alone in the dark on one side or the other. 

It gets more complicated (of course):
1. Person A takes 1 minute to cross 
2. Person B takes 2 minutes to cross 
3. Person C takes 5 minutes to cross 
4. Person D takes 10 minutes to cross

Your job, if you accept the challenge, is to get everyone across the bridge, never more than two on the bridge at a time, one person always carrying the lantern on the bridge, in under 18 minutes.


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## davesweep (Nov 23, 2012)

One way could be

A and B cross 
A returns
C and D cross 
B returns
A and B cross


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## chuckchuckit (Nov 23, 2012)

A and D cross.
A returns with lantern = 11 minutes so far.
B and C cross = 16 minutes and they all have crossed the bridge.

Since you did not require they all had to end up on the other side. They have all crossed the bridge at least once, including A who is back on the original side though (in the dark perhaps saying "toss me the lantern..."). Only 16 minutes were needed.


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## hiker95 (Nov 23, 2012)

davesweep,

Nicely done.

It took 17 total minutes to get everyone onto the other side of the very, very, deep gorge.


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## chuckchuckit (Nov 23, 2012)

Nice work again by Dave, getting everyone to the other side. Without anyone having to toss anyone the lantern.


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