Bridge Challenge

[Ana the Ist]

I wanna tell everyone how I got it and look smart!!!

 

[Tanj]

15 minutes.

 

[Shemjaza]

I feel like it's 17, so I'm curious of the techniques that her it lower.

 

[VirOptimus]

15

 

[Ana the Ist]

I feel like it's 17, so I'm curious of the techniques that her it lower.

I actually got 17 as well, I was just messing around with the 12 stuff. There's a way to get 10 if the 8+5 were on opposite sides of the bridge from the 1+2...but OP edited it to make clear they all start on the same side.

 

[Ana the Ist]

Unless it doesn't matter which side they end up on, as long as they all crossed....then its 11.

 

[Sabertooth]

I say 17 minutes.

I can now think of a possible 15 minute solution.

 

[ThievingMagpie]

15 i thiiiiink

 

[DogmaHunter]

First rule of bridge challenge: Your first response to the bridge challenge is just to reply with your answer in terms of minutes, no other explanation of details. Other comments not directly related to your solution are okay.

Second rule of bridge challenge: later in thread, we'll open it up to larger discussion.

Third rule of bridge challenge: No this doesn't have much to do with Physical & Life Sciences, but I may be able to make an analogy later that will bring it closer to topic.

Bridge situation:

An improbably rickety bridge can only be crossed by at most two people at a time.
It is also dark and there may be grues on the bridge. You need a torch to cross the bridge (but both sides are safe).
There is only one torch. (so when it is carried across, someone will have to bring it back)

Four people need to cross the bridge (and all 4 start on the same side):
A speedy person who can cross the bridge in 1 minute.
A fit person who can cross the bridge in 2 minutes.
A soft person who can cross the bridge in 5 minutes.
An infirm person who can cross the bridge in 8 minutes.

When two people travel together, they can only go as fast as the slower person. (e.g. if speedy and infirm go together, it takes 8 minutes to cross).

Bridge challenge:

What is the shortest amount of time required to get all 4 people across the bridge?

Last rule of bridge challenge: See First Rule! First answers should only provide the time in minutes, not a detailed description of the solution.

Perhaps the solution is already given by now.
I'm gonna go with 15 minutes and proceed reading the thread

EDIT: nope, no solution yet. Sticking to 15. Don't see how it could be done faster.

 

[ThievingMagpie]

In the words of Ron Swanson: "I absolutely do not want to solve a series of riddles and clues each more intricate than the last - do you understand what I'm saying... "

"...Uh, I do want that, please do that for me."

 

[essentialsaltes]

As an American, do you mean a torch as in a burning piece of wood or a flashlight?

In the context of the challenge, it makes no difference.

As an American, I was somewhat confused as a child when one of the Narnia kids had 'an electric torch'.

 

[essentialsaltes]

Thanks, people, for playing along so far.

I don't want to toy with you all for too long, but just to clarify some issues and sum up.

The first 4 numerical answers were 17 minutes. Followed by a couple 10's, a 19 and more recently some 15's have popped up, including Sabertooth who has switched from 17 to 15.

Just to make it clearer, the 4 people start on one side, and end on the other side of the bridge. And because of the torch, there has to be some back and forth. So if our 4 people are A, B, C, D and the torch is * and the bridge looks like ____________, there are several stages involving at least 5 complete crossings. One sample pattern looks like this

ABCD* ____________
CD _____A*B_______ (2 people go right)
CD _____A*_______ B (1 person goes left to take the torch back)
ACD* ____________ B
D _____A*C_______ B (2 people go right)
D _____A*_______ BC (1 person goes back)
AD* ____________ BC
_____A*D_______ BC (2 people go right)
____________ ABCD*

Since there are at least 5 crossings, and we know the 8 minute person will take 8 minutes to go across...

It's not giving too much away to say that the optimal solution involves the 8 minute person only crossing once! But since the shortest possible crossing is 1 minute, a very crude estimate for the minimal crossing time is 8 minutes for the slow person and 4 minutes for the 4 other crossings, which makes 12 minutes in all.

So without going deeper into discussion, I just want to leave time for the 10 minute answer people (or anyone really) to reconsider.

 

[Sanoy]

Whats a grue? And can I set someone on fire with the torch thereby multiplying my anti grue devices.

 

[durangodawood]

17

(and, yes, whats a "grue"?

 

[essentialsaltes]

Whats a grue? And can I set someone on fire with the torch thereby multiplying my anti grue devices.

Grues are monsters that I know best from the old Infocom Text Adventures like Zork (though I guess the name is borrowed from fiction by Jack Vance).

"Zork's grues fear light and devour human adventurers, making it impossible to explore the game's dark areas without a light source. ... A common catchphrase associated with grues is the line that displays whenever players in Zork and related Infocom games enter a dark area without a light source: "It is pitch black. You are likely to be eaten by a grue.""

Setting people on fire will not help. Although obviously this is something of a puzzle, the solution is relatively straightforward and does not require that kind of out of the box thinking. Also, shame on you.

 

[Sanoy]

Grues are monsters that I know best from the old Infocom Text Adventures like Zork (though I guess the name is borrowed from fiction by Jack Vance).

"Zork's grues fear light and devour human adventurers, making it impossible to explore the game's dark areas without a light source. ... A common catchphrase associated with grues is the line that displays whenever players in Zork and related Infocom games enter a dark area without a light source: "It is pitch black. You are likely to be eaten by a grue.""

Setting people on fire will not help. Although obviously this is something of a puzzle, the solution is relatively straightforward and does not require that kind of out of the box thinking. Also, shame on you.

Ah I knew it sounded familiar. I played the second Zork, return to Zork. I kept getting stuck and just trying to use everything on anything. Sometimes it results in hilarious game responses. I wonder if I can get through it now.

 

[durangodawood]

....But since the shortest possible crossing is 1 minute, a very crude estimate for the minimal crossing time is 8 minutes for the slow person and 4 minutes for the 4 other crossings, which makes 12 minutes in all.....

Um, what?

 

[Chesterton]

So without going deeper into discussion, I just want to leave time for the 10 minute answer people (or anyone really) to reconsider.

Are we allowed to discuss, or do you want to wait for more answers?

Anyway, I'm changing my answer to 10 minutes.

 

[DogmaHunter]

Thanks, people, for playing along so far.

I don't want to toy with you all for too long, but just to clarify some issues and sum up.

The first 4 numerical answers were 17 minutes. Followed by a couple 10's, a 19 and more recently some 15's have popped up, including Sabertooth who has switched from 17 to 15.

Just to make it clearer, the 4 people start on one side, and end on the other side of the bridge. And because of the torch, there has to be some back and forth. So if our 4 people are A, B, C, D and the torch is * and the bridge looks like ____________, there are several stages involving at least 5 complete crossings. One sample pattern looks like this

ABCD* ____________
CD _____A*B_______ (2 people go right)
CD _____A*_______ B (1 person goes left to take the torch back)
ACD* ____________ B
D _____A*C_______ B (2 people go right)
D _____A*_______ BC (1 person goes back)
AD* ____________ BC
_____A*D_______ BC (2 people go right)
____________ ABCD*

Since there are at least 5 crossings, and we know the 8 minute person will take 8 minutes to go across...

It's not giving too much away to say that the optimal solution involves the 8 minute person only crossing once! But since the shortest possible crossing is 1 minute, a very crude estimate for the minimal crossing time is 8 minutes for the slow person and 4 minutes for the 4 other crossings, which makes 12 minutes in all.

So without going deeper into discussion, I just want to leave time for the 10 minute answer people (or anyone really) to reconsider.

Sticking to 15.

 

[essentialsaltes]

Um, what?

Just as a crude estimate. I'm trying to suggest that the people saying 10 minutes must have misunderstood something. Since even this crude estimate shows the answer must be greater than 12. THe answer is certainly not 12 minutes.