Bridge Challenge

[AV1611VET]

10

 

[Strathos]

I don't get how it could be 15...

 

[AirPo]

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.

17


Did this come out of the HearthStone Puzzle Lab?

 

[Chesterton]

I'm trying to suggest that the people saying 10 minutes must have misunderstood something.

Or maybe you haven't thought of something.

It's 10.

 

[durangodawood]

Or maybe you haven't thought of something.

It's 10.

 

[trunks2k]

15 minutes.

 

[trunks2k]

I don't get how it could be 15...

Shoot me a PM and I'll explain it.

 

[Sabertooth]

...which makes 12 minutes in all.

Using your terms, B, C & D can be in any order.
A= 1 min
B= 2
C= 5
D= 8

Spoiler: For 17

_2 min, A*B>
_1 min, <A*
_5 min, A*C>
_1 min, <A*
+8 min, A*D>
17 mins, total

Spoiler: For 15

_2 min, A*B> (A* stops in the middle, because the ends are safe)
_5 min, C> (C on bridge when B exits)
+8 min, D> (D on bridge when C exits. A* accompanies D upon meeting.)
15 mins, total

 

[essentialsaltes]

No, no, grues are far too vicious to be tricked by people separating on the bridge and doing complicated things like that. People wandering down half-bridges without a torch will be eaten.

When pairs of people go, they go together across the whole bridge.

 

[AV1611VET]

C & D cross first, scaring away those monsters and clearing the bridge.

Then A & B cross.

Total time: 10 minutes.

 

[Occams Barber]

C & D cross first, scaring away those monsters and clearing the bridge.

Then A & B cross.

Total time: 10 minutes.

You forgot about the torch.
OB

 

[trunks2k]

Using your terms, B, C & D can be in any order.
A= 1 min
B= 2
C= 5
D= 8

Spoiler: For 15

_2 min, A*B> (A* stops in the middle, because the ends are safe)
_5 min, C> (C on bridge when B exits)
+8 min, D> (D on bridge when C exits. A* accompanies D upon meeting.)
15 mins, total

No, that's not in line with the rules.

Spoiler: Real spoiler for 15

A and B go across the bridge (2 minutes)
A comes back with torch (1 minute)
C and D cross the bridge (8 minutes)
B comes back with torch (2 minutes)
A and B go back across the bridge (2 minutes)

2+1+8+2+2 = 15.

 

[Chesterton]

You forgot about the torch.
OB

A and B don't need the torch. C and D signal or call out "No grues. Come on over, guys."

 

[essentialsaltes]

Grues do not scare easily. They infest the area.

OK, people are champing at the bit here. So I release you all from your vows of silence.

I'm really intrigued by the early support for 17, and the late support for 15 (and the Discordian support for other options.)

But before people read further where all is revealed.,,

To the people on team 17 minutes, I now assert that the answer is in fact 15 minutes.

Before you read further, one or two questions.

Does hearing now that the answer is definitely 15 (because essentialsaltes says so, and this is my challenge) encourage you to look again?
[Did seeing other people say 15 earlier in the thread encourage you to look again?]

 

[durangodawood]

Time wasted thinking of the real 15 min solution: 20 minutes.

 

[durangodawood]

.....Did seeing other people say 15 earlier in the thread encourage you to look again?

Yes it did.

But I was still too dumb to get the 15 min solution.

 

[Chesterton]

Grues do not scare easily. They infest the area.

But do they have the ability to magically appear behind you on a bridge you've just crossed?

 

[essentialsaltes]

But do they have the ability to magically appear behind you on a bridge you've just crossed?

They just come out of the dark, man.

That's it, man. Game over, man. Game over!

 

[Occams Barber]

Grues do not scare easily. They infest the area.

OK, people are champing at the bit here. So I release you all from your vows of silence.

I'm really intrigued by the early support for 17, and the late support for 15 (and the Discordian support for other options.)

But before people read further where all is revealed.,,

To the people on team 17 minutes, I now assert that the answer is in fact 15 minutes.

Before you read further, one or two questions.

Does hearing now that the answer is definitely 15 (because essentialsaltes says so, and this is my challenge) encourage you to look again?
[Did seeing other people say 15 earlier in the thread encourage you to look again?]

I looked again at all the answers that weren't 17 minutes but still kept coming back to 17. I suspect that the early posts all agreeing on 17 sorta convinced me that I must be right.

It took trunk2k's explanation in post 52 before I realised how wrong I was.
OB

 

[Sabertooth]

Does hearing now that the answer is definitely 15 (because essentialsaltes says so, and this is my challenge) encourage you to look again?
[Did seeing other people say 15 earlier in the thread encourage you to look again?]

Yes, 17 seemed too easy/straightforward.