evil bridge problem

PondScm007 · Posted: Wed Mar 05, 2003 10:31 am Post subject: evil bridge problem

NJ has high-school standardized testing this week, and in the time after whilst being stuck sitting in class for liek a half hour waiting for everyone throughout the school to finish (gah!) somone told me this problem... allegedly it IS possible to do, but i cant figure it out...anyone have any better luck?

there are four people- 1, 2, 5, and 10. they have to all get across a bridge.
the bridge can only hold 2 people at a time. someone walking on the bridge must be
carrying a flashlight. the people's names correspond to how long it takes
them to cross (1 takes 1 minute to get across, 5 takes 5 minutes to get across etc)
they must all cross in 17 minutes. if two people cross the bridge, they move at
the slower person's rate (so if 1 and 5 went across, it would take 5 minutes)
how do they do it?

????????????????????????????
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BR, yo

Thunder · Registered User Joined: 08 May 2002 Posts: 77

-10 and 1 (1 has flash light) cross together and 10's speed.
-1 turns back to start point and 5 begins to cross toward other side where 10 now is (both of course moving at 5's speed).
-1 turns back around with 2 in tow and bothe cross together at 2's speed.

10 + 5 + 2=17 minutes and all across.

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The Nanite · Posted: Wed Mar 05, 2003 11:06 am Post subject:

Why can't they all have flashlights?

1 and 10 cross at the same time = 10 minutes
2 goes alone = 2 minutes
5 goes alone = 5 minutes

total time = 17 minutes.
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Usually Dead · Posted: Wed Mar 05, 2003 11:07 am Post subject:

There was another problem like this I heard about in grade school. It went something like this:

A farmer wants to take his dog, goose, and a head of lettuce across the river to sell at the marketplace. There's a small boat to get across river, but there's only enough room for the farmer and one of his items at a time.
The farmer can't leave the goose alone with the head of cabbage, becuase the goose will eat it. He can't leave the dog alone with the goose, becuase the dog will eat that poor bird. So how does he get himself and all his things across the river intact?

The answer was this:

The farmer takes the goose -----------> over the river and drops it off on the far side. It's okay to leave the dog alone with the cabbage since dogs are carnivores.
Then the farmer comes back <------------- and gets the dog. He takes the dog over ----------> to the far side of the river and drops the dog off, then gets the goose and comes back again <----------- to the side of the river he started on. Now the farmer, the goose, and the cabbage are on the side of the river he started on. The dog is alone on the far side.

The farmer takes the cabbage --------> over to the far side of the river and sets it next to the dog, leaving the goose alone. Then he comes back
<--------- and gets the goose, then goes to the far side one last time --------->. So now he finally has the dog, goose and cabbage all on the marketplace side of the river. Yay!

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sharkbyte · Posted: Wed Mar 05, 2003 2:34 pm Post subject:

sharkbyte · Posted: Wed Mar 05, 2003 2:38 pm Post subject: Re: evil bridge problem

The Nanite · Posted: Wed Mar 05, 2003 2:41 pm Post subject: Re: evil bridge problem

sharkbyte · Posted: Wed Mar 05, 2003 2:45 pm Post subject: Re: evil bridge problem

Fred Astaire · Posted: Wed Mar 05, 2003 2:55 pm Post subject:

ok now lets assume they have one flashlight and have to share it.

1+2 cross= 2 mins
1 comes back= 3 mins
5+10 cross= 13 mins
this time 2 comes back= 15 mins
1+2 cross= 17 mins

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for Blue

Thunder · Registered User Joined: 08 May 2002 Posts: 77

I believe 17 minutes. For this to be a problem at all I think you have to go with the "one flashlight".....

*****
Also, when one begins his trip back, 5 is going at the same time in the other direction:

5 --------->
1 <---------

So you only count 5 min., they are crossing at the same time....just in opposite directions. The whole idea is to get you to think of a clever way to solve the puzzle.

*****
Everyone having a flashlight doesn't make since....if evryone had one then why even mention it at all.

_________________
I put on my robe and Wizard's hat!!! HARRRRR

Thunder · Registered User Joined: 08 May 2002 Posts: 77

Also before you write back and tell me that 5 will not be able to see because 1 is going the other direction with the flashlight....don't.

If you read the prob. it says: "someone walking on the bridge must be
carrying a flashlight". That is all it says about the flashlight....and it makes for a good clue as to the solution.

And that's about all i got 2 say abot that.

Your friend,
-Forest
_________________
I put on my robe and Wizard's hat!!! HARRRRR

CyC0Dad · Posted: Wed Mar 05, 2003 5:14 pm Post subject:

PondScm007 · Posted: Wed Mar 05, 2003 5:17 pm Post subject:

neg you can only be walking if you have the flashlight, or the person next to you has the light.. and i found it out a bit later, i gave in and begged for the answer...gj fred and dad
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BR, yo

Replica · Posted: Mon Mar 10, 2003 12:51 pm Post subject:

this reminds me ( not near as hard as) of the ap calc test last year... man some fo those problesm sucked. got a lousy 3 on it, lol. but not that this school takes them anyways
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