# Bridge-crossing puzzles



## robert@fm (Apr 29, 2016)

1)
Four men (Andy, Bill, Chas and Dave) are making a journey at night when they come across an old, dilapidated bridge which they must cross. Unfortunately they have only one torch between them, and the bridge is in such a bad condition that it cannot safely be crossed without the torch (which must be carried across the bridge), nor can it support more than two of them at a time; and none of them is up to carrying another.

If it takes A one minute to cross the bridge, B two minutes, C five and D ten, what is the shortest amount of time for all four to cross?

2)
You come across a bridge which will take you 20 minutes to cross. Unfortunately it is guarded by an evil guard who checks it every 15 minutes; if he finds anyone crossing it he orders them to turn back (and watches to make sure they comply), and if they refuse he shoots them dead.

You absolutely must cross this bridge, and there's no way to prevent the guard watching (nor anywhere to buy a bullet-proof vest); how do you do it?


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## Alan.tnh (Apr 29, 2016)

2 )walk across for 14 mins, then turn around and walk back for 1min ne will catch you and send you back the way you wanted to go.


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## Annette (Apr 29, 2016)

1) 19 minutes and a very tired Andy?


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## Alan.tnh (Apr 29, 2016)

Annette Anderson said:


> 1) 19 minutes and a very tired Andy?


I think Andy should check his BG level...........


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## robert@fm (Apr 29, 2016)

Annette Anderson said:


> 1) 19 minutes and a very tired Andy?


Actually, it can be done faster than that (and with a bit less legwork on Andy's part).


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## Annette (Apr 29, 2016)

If they've got a rope and roller skates, then 5 minutes


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## Annette (Apr 29, 2016)

Does the torch have to be carried by the person crossing the bridge? ie, if one is crossing one way and one the other, can one of them carry the torch and light up the whole bridge for the other?


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## robert@fm (May 25, 2016)

Nobody has got the correct solution to the first one, hence bumping.


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## Matt Cycle (May 25, 2016)

I know this one.  Although it had to be explained to me.  It's to do with getting the two slowest over at the same time. 

A and B cross - 2 minutes
A returns - 1 minute
C and D cross - 10 minutes
B returns - 2 minutes
A and B cross - 2 minutes

Total 17 minutes and equally tired Andy and Bill whilst Chas and Dave were probably sat about rabbiting to each other.


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