wu :: forums
        (http://www.ocf.berkeley.edu/~wwu/cgi-bin/yabb/YaBB.cgi)
      

        riddles >> hard >> High buildings and street vendors solution >=P
        
(Message started by: Luis Edgardo Argote Bolio on Jun 16th, 2005, 5:11am)
Title: High buildings and street vendors solution >=P
Post by Luis Edgardo Argote Bolio on Jun 16th, 2005, 5:11am
The solution to this one is remarkably simple:

<img src="http://xs33.xs.to/pics/05244/tsctn.PNG">
http://xs33.xs.to/pics/05244/tsctn.PNG

(wasn't sure if this forun uses BB code or HTML)

Here's the direct link too: http://xs33.xs.to/pics/05244/tsctn.PNG
Title: Re: High buildings and street vendors solution >
Post by ArtOf_War on Jun 16th, 2005, 5:14am
OK so i just registered so I could track the topic... if anyone has any obsevations on the solution please let me know

and here's the original problem: HIGH BUILDINGS AND STREET VENDORS

>=P      

Consider a country in which people live in high buildings and buy things from street vendors, using some form of money. Each building is equipped with a pulley in the eaves over which runs a long rope with a basket on it, operable by either party. Is there a system by which a resident and a vendor who don't trust each other can conduct trade? Note that either party may plan to play the opposite role with someone else later.
Title: Re: High buildings and street vendors solution >
Post by Grimbal on Jun 16th, 2005, 5:19am
And what if the buyer discovers the goods are not what he ordered, or not the right quantity?  Or if the seller discovers Monopoly money in the basket?
Title: Re: High buildings and street vendors solution >
Post by ArtOf_War on Jun 16th, 2005, 5:20am
ok so the entire answer is wrong... or at least needs some rethinking...
Title: Re: High buildings and street vendors solution >
Post by towr on Jun 16th, 2005, 6:34am
I think it would be difficult to address those extra concerns. There's pretty much always a way for one party to fool the other.
The buyer could simply drop an anvil on the vendor and steal all his merchandise. :P
Title: Re: High buildings and street vendors solution >
Post by Grimbal on Jun 16th, 2005, 7:00am
The street vendor can run away.  The buyer can not.

The seller just delivers the goods first and gets the money afterwards.  If the money doesn't come, he can call a few pals, go up and get the money plus recovery charges.  Or he can instruct the other street vendors not to sell to that guy any more and let him starve.
Title: Re: High buildings and street vendors solution >
Post by ArtOf_War on Jun 16th, 2005, 12:32pm
well there could be cameras and monitors one every level so wehn the transaction occurs both parties can see what they are going to recieve
Title: Re: High buildings and street vendors solution >
Post by SMQ on Jun 16th, 2005, 2:16pm
Or puth the merchandise and the payment could be in plexiglass boxes padlocked to the rope with keys attached further along on the rope.  Winch the rop to the boxes, both parties verify their satisfaction, winch to the keys then back to the boxes, both parties open their respective boxes.  Only works if either party can successfully prevent the rope from being winched to the keys in the event of their dissatisfaction--maybe a one-way brake at each point...

;D

-SMQ
Title: Re: High buildings and street vendors solution >
Post by paul schmitz on Jun 30th, 2005, 4:51am
as stated there is no way to conduct fair trade assuming that the only way the people can interact is by using the pully system.  either party could stiff the other, and knowing this, no one would enter a trade.  if you assume naturally good people that want to facilitate a strong economy, then everyone gains the most benefit by fulfilling all trades.  however, since the puzzle supposes that the people dont trust eachother, i think it also assumes some kind of fear construction and that leads to stealing.

in short: any amount of anything put into any basket could be taken and the person losing it could recieve nothing, so no one will ever begin a trade
Title: Re: High buildings and street vendors solution >
Post by paul schmitz on Jun 30th, 2005, 5:05am
every person steals the key after the trade is done, then whoever had to buy the key/lock in the first place has lost additional goods (the key).  they could charge additional money to pay for the key and include the key for free.  then, one person will steal one key from every trade, and when the roles switch, they will be able to provide one key to their own mechanism which has had one key stolen from it, thus making it functional again!
i dont think it is possible if you assume there is only one thing attached to the pulley from each person (ie. no keys).
Title: Re: High buildings and street vendors solution >
Post by thecrazydreamer on Jun 20th, 2006, 2:12pm
Here is my solution:

The vendor gets in the basket, and the resident pulleys him/her up.  The exchange is made face to face.

If the vendor is unhappy (s)he refuses to leave.  If the resident is unhappy (s)he refuses to pulley the vendor back to the street.
Title: Re: High buildings and street vendors solution >
Post by Aravis on Jul 13th, 2006, 12:00pm
This seems to me like the classic public key access cryptography system.

Take a pulley set with a plexiglass safe at each end (assume the safes and keys and padlocks are not pickable, breakable, etc.)  Each safe can be padlocked with two padlocks simultaneously, but one lock is sufficient to prevent entry.  Both parties must winch together to move the pulley.

Each party puts their goods in one of the safes, and locks it with their own padlock.  They not move the pulley, so that each is looking at the others goods.  Once they are satisfied that the goods are real, each puts a padlock on this safe as well.  At this point, each safe is padlocked with both peoples padlocks.

They then move the pulley again, so that each is seeing their own goods.  They then remove their padlock.  Now, the safe containing the goods person A wants to trade to person B is locked with person B's lock only, and vice versa.

Now they move the pulley again, and unlock their own padlock on the chest containing the other's goods.  This was the last padlock on the safe, so they can take the goods out.  Now each has the others goods, which was the point of the trade, and at no point could the other person steal goods, padlocks, or keys, nor could anyone move the safes without agreement.

I'm pretty sure that solves the problem.
Title: Re: High buildings and street vendors solution >
Post by rmsgrey on Jul 15th, 2006, 9:04am

on 07/13/06 at 12:00:17, Aravis wrote:
This seems to me like the classic public key access cryptography system.

Take a pulley set with a plexiglass safe at each end (assume the safes and keys and padlocks are not pickable, breakable, etc.)  Each safe can be padlocked with two padlocks simultaneously, but one lock is sufficient to prevent entry.  Both parties must winch together to move the pulley.

Each party puts their goods in one of the safes, and locks it with their own padlock.  They not move the pulley, so that each is looking at the others goods.  Once they are satisfied that the goods are real, each puts a padlock on this safe as well.  At this point, each safe is padlocked with both peoples padlocks.

They then move the pulley again, so that each is seeing their own goods.  They then remove their padlock.  Now, the safe containing the goods person A wants to trade to person B is locked with person B's lock only, and vice versa.

Now they move the pulley again, and unlock their own padlock on the chest containing the other's goods.  This was the last padlock on the safe, so they can take the goods out.  Now each has the others goods, which was the point of the trade, and at no point could the other person steal goods, padlocks, or keys, nor could anyone move the safes without agreement.

I'm pretty sure that solves the problem.

What happens if one person fails to remove their first padlock?
Title: Re: High buildings and street vendors solution >
Post by Aravis on Jul 15th, 2006, 11:01am
That's a real good question.  I guess one idea would be that you should be able to count how many locks are on the safe from a relatively large distance, as the safes are clear, so that each particiant in the trade could see whether the other had been faithful in removing the lock.  If one had not removed the lock, then the other refuses to help move the pulley the rest of the way, so the issue could be resolved by returning the unlocked safe to the unfaithful party.

This would be easier if the locks were really big, and maybe if they were brightly painted, so that one could easily see them from a distance.
Title: Re: High buildings and street vendors solution >
Post by Grimbal on Jul 17th, 2006, 2:32pm
Well, then you are almost back to the situation where there are no locks, only a clear case.  They look at the box from a safe distance, and only if they agree with the content they let the puley go the last meter.

And the fact that the locks are big and bright won't prevent one party to replace it with a small dark lock.
Title: Re: High buildings and street vendors solution >
Post by Aravis on Jul 17th, 2006, 4:51pm
if the latch points are positioned correctly, it should be easy to spot a lock when there is not supposed to be one, whereas it could be very difficult to spot bad merchandise without examining it closely, thus neccessitating the ability to look at he merchandise at a short dstance for an extended period of time.
Title: Re: High buildings and street vendors solution >
Post by BNC on Jul 17th, 2006, 11:34pm
I'd like to suggest a non-math method, that seem to work in the movies...

The resident tears a note of money in half, and places the left side in the busket, thereby loosing the money himself (no motive to cheat anymore) but giving the vendor no actual money (no motive to cheat either).

Then the vendor places the goods in the basket. The resident has already paid for them from his point of view (as he can't use the half-money again), so there's no reason not to place the other half afterward.
Title: Re: High buildings and street vendors solution >
Post by rmsgrey on Jul 18th, 2006, 9:01am

on 07/17/06 at 23:34:40, BNC wrote:
I'd like to suggest a non-math method, that seem to work in the movies...

The resident tears a note of money in half, and places the left side in the busket, thereby loosing the money himself (no motive to cheat anymore) but giving the vendor no actual money (no motive to cheat either).

Then the vendor places the goods in the basket. The resident has already paid for them from his point of view (as he can't use the half-money again), so there's no reason not to place the other half afterward.

1) The buyer would have no motive not to cheat either, so there'd be no reason to place the other half afterward

2) If everyone adopts that system, then the half note can be used later to pay for another transaction, giving it value after all.

3) Here in the UK, every banknote has two copies of the serial number on it. The Bank of England treats any fragment with a valid serial number on it as worth half the notional value of the original note. In such a system, both halves have an independent value, so either party can profitably cheat...
Title: Re: High buildings and street vendors solution >
Post by MorbidJoe on Jul 24th, 2006, 2:25pm

on 07/17/06 at 23:34:40, BNC wrote:
I'd like to suggest a non-math method, that seem to work in the movies...

The resident tears a note of money in half, and places the left side in the busket, thereby loosing the money himself (no motive to cheat anymore) but giving the vendor no actual money (no motive to cheat either).

Then the vendor places the goods in the basket. The resident has already paid for them from his point of view (as he can't use the half-money again), so there's no reason not to place the other half afterward.


Spite  ;D

Anyway, in answer to rmsgrey's points..

1) It doesn't say they are nasty people, just that they don't trust each other. Therefore, he most likely would send the other half of the money.

2) I'll get back to you on that  :P

3)Don't send the bit with the serial number on.
Title: Re: High buildings and street vendors solution >
Post by BNC on Jul 24th, 2006, 11:40pm

on 07/18/06 at 09:01:19, rmsgrey wrote:
1) The buyer would have no motive not to cheat either, so there'd be no reason to place the other half afterward

2) If everyone adopts that system, then the half note can be used later to pay for another transaction, giving it value after all.

3) Here in the UK, every banknote has two copies of the serial number on it. The Bank of England treats any fragment with a valid serial number on it as worth half the notional value of the original note. In such a system, both halves have an independent value, so either party can profitably cheat...


1) You have a point here. There is no information to decide what the resident will do if he may cheat, but have no gain from cheating. I think most people (even dishonest ones) would not cheat, but hey -- we're in riddle land.
One possible (although complicated) solution is to issue a personal seal for each resident, with which he must stamp his money (it would have to fade after a while). The vendors will keep a "don't trust" list of residents with whom no trade should take place. So, by cheating the resident will have something to loose, but nothing to gain.

2) Easily solved. The resident must first place both halves of the note into a sealed box, lower it for inspection, and then raise it up again before sending one half down. This way half note will have no value.

3) That place is arguably not in the UK, so UK rules not necessarily apply.
Title: Re: High buildings and street vendors solution >
Post by Sjoerd Job Postmus on Aug 2nd, 2006, 8:43am

on 07/13/06 at 12:00:17, Aravis wrote:
This seems to me like the classic public key access cryptography system.

Take a pulley set with a plexiglass safe at each end (assume the safes and keys and padlocks are not pickable, breakable, etc.)  Each safe can be padlocked with two padlocks simultaneously, but one lock is sufficient to prevent entry.  Both parties must winch together to move the pulley.

Each party puts their goods in one of the safes, and locks it with their own padlock.  They not move the pulley, so that each is looking at the others goods.  Once they are satisfied that the goods are real, each puts a padlock on this safe as well.  At this point, each safe is padlocked with both peoples padlocks.

They then move the pulley again, so that each is seeing their own goods.  They then remove their padlock.  Now, the safe containing the goods person A wants to trade to person B is locked with person B's lock only, and vice versa.

Now they move the pulley again, and unlock their own padlock on the chest containing the other's goods.  This was the last padlock on the safe, so they can take the goods out.  Now each has the others goods, which was the point of the trade, and at no point could the other person steal goods, padlocks, or keys, nor could anyone move the safes without agreement.

I'm pretty sure that solves the problem.

Which has a solution quite similar to another popular riddle...
http://www.onlyriddles.com/351/Valuable-Object.html

Except you now have two senders.
Title: Re: High buildings and street vendors solution >
Post by logan on Nov 28th, 2006, 4:25am
Sjoerd Job Postmus a nice solution of Pk. how about a third party then? A neighbor with no profit or loss in this action may help. place the money and the order to the basket lift it to the third party, let him/her take the money and sent order to the vendor. Vendor puts the stuff in the basket sends it up with order sheet let third party check the goods with ordersheet and send it to consumer. than third party 'd sent the money to the vendor. third party can't trick both sides becouse

1-later someother neigbor/the vendor may trick him/her.
2-prestige issues.  :P
Title: Re: High buildings and street vendors solution >
Post by Icarus on Nov 28th, 2006, 4:57pm
But everyone here already knows everyone else is a greedy thief, so they have no prestige to lose. And if anyone can come up with a system that does not require depending on a third-party, he can steal the money or goods of any neighbor foolish enough to trust him, and never have to expose himself to retaliation.

Better to come up with a no-third-party solution in the first place.
Title: Re: High buildings and street vendors solution >
Post by Product on Jun 1st, 2007, 12:35pm
Okay, I think I've found a solution. I won't get too excited though, because in my short time on these forums every time I've said that I've been wrong.

The pulley system is sophisticated. Both plexiglass safes are sealed tight with attached gloves (that cannot be removed or ripped) so that each party can check out the merchendise beforehand. They are locked with computerized combination locking mechanisms that can't be hacked or shorted out. The boxes are connected to a main processing unit in the middle of the building, and each man has three buttons that he can push.

The vendor and buyer agree on a price by yelling to one another. The two parties put their respective items in the safes and input a ten digit code of their choosing, which they write down. They then press the 'transfer' button. When BOTH buttons are pressed at the same time, the safes switch positions. When the safes arrive, they hit the 'lock pulley' button. Either button can stop any movement of the pulley. Each party can check out the goods for as long as they want. When each is satisfied, they hit the 'combination' button. When a lock button has been pushed in, AND both parties are holding the 'combination' button, the safes communicate through the central unit and switch combinaitons. Each party can then look at the 10 digit code he wrote down, enter it, and take the goods. Once the safes are shut, the combinations are erased and the safe can be freely open and shut, ready for the next transaction.

I can't think of a way to cheat it.
Title: Re: High buildings and street vendors solution >
Post by malchar on Jan 27th, 2009, 11:54am
Of all the hard riddles, I could never get over this one until now. Bumping this thread to show that it's impossible to solve. If any kind of safe were to be used to encase goods, it would be impossible to be sure that the goods are legitimate. If the safes are translucent, the goods could be faulty. For example, just looking at electronics doesn't verify their intengrity. Even if the safes are "interactive" i.e. they have gloves attached, there are still theoretical ways that the goods could be illegitimate. Basically, if you don't have a full ability to grasp and inspect something without any boundaries, then it could be fake, and if all boundaires are removed, you could steal it.

Of course, in real life we buy things like this all the time. It is only because of the rule of law that we are willing to assume that goods are legitimate until proven otherwise. The only solution that I can think of would require some kind of police system.

Even the best answers so far are far from perfect. In the case of cutting money in half, one person could choose to sacrifice their goods in order to deny the other person the ability to get their goods back. Further, in any kind of rope system, one person could simply put nothing in their basket, swap baskets, and then hold onto the rope forever, preventing the other person from getting their items back.
Title: Re: High buildings and street vendors solution >
Post by Grimbal on Jan 27th, 2009, 1:43pm

on 01/27/09 at 11:54:19, malchar wrote:
Of course, in real life we buy things like this all the time. It is only because of the rule of law that we are willing to assume that goods are legitimate until proven otherwise. The only solution that I can think of would require some kind of police system.

I think it is enough if both parties have an interest in good business relations in the long term.  Both will comply to the deal just to be sure they will continue to buy/sell from each other.


Powered by YaBB 1 Gold - SP 1.4!
Forum software copyright © 2000-2004 Yet another Bulletin Board