Anyone have any good riddles or brain-teasers to share?

[BigJ]

Originally posted by: Glavinsolo
Here is one:
A woman parallel parks a car in between two other cars.
How is this possible?

Because she waves down a man walking down the street to do it for her.

 

[Born2bwire]

Originally posted by: Glavinsolo

Originally posted by: Atheus
Einstein's riddle. Apparently only 2% of the population can solve it.

1. In a street there are five houses, painted five different colours.
2. In each house lives a person of different nationality
3. These five homeowners each drink a different kind of beverage, smoke different brand of cigar and keep a different pet.

Hints:

1. The Brit lives in a red house.
2. The Swede keeps dogs as pets.
3. The Dane drinks tea.
4. The Green house is next to, and on the left of the White house.
5. The owner of the Green house drinks coffee.
6. The person who smokes Pall Mall rears birds.
7. The owner of the Yellow house smokes Dunhill.
8. The man living in the centre house drinks milk.
9. The Norwegian lives in the first house.
10. The man who smokes Blends lives next to the one who keeps cats.
11. The man who keeps horses lives next to the man who smokes Dunhill.
12. The man who smokes Blue Master drinks beer.
13. The German smokes Prince.
14. The Norwegian lives next to the blue house.
15. The man who smokes Blends has a neighbour who drinks water.

WHO OWNS THE FISH?

If I sat down for a couple of hours and had my discrete mathematics book out I could solve it.

If you guys want I could post the worksheet I did. But really, you make up the logic grid table and work it through. The only trick is that you have to make a separate layout of the houses to show their relationship along the street. Once you do that, you can quickly work out from the locations of the houses and their information what goes with what.

 

[cKGunslinger]

Originally posted by: Glavinsolo
Here is one:
A woman parallel parks a car in between two other cars.
How is this possible?

The other cars are 3+ miles apart?

 

[allisolm]

The only trick is that you have to make a separate layout of the houses to show their relationship along the street.

Exactly. I had a page with house 1,2,3,4,5 and a logic problem grid page and I only did it wrong and had to start over once. There's nothing worse when doing one of these to get 3/4 the way through and realize you've got an error.

 

[Garet Jax]

Originally posted by: allisolm
The only trick is that you have to make a separate layout of the houses to show their relationship along the street.

Exactly. I had a page with house 1,2,3,4,5 and a logic problem grid page and I only did it wrong and had to start over once. There's nothing worse when doing one of these to get 3/4 the way through and realize you've got an error.

Actually there is an infinite number of things worse:

You could get 7/8, 15/16, 31/32 .... of the way through before realizing the error

 

[thraashman]

Ok, the 9 balls riddle. I don't know if anyone posted this answer yet, doesn't look like it though I only skimmed through

I'll do this in weighings.

#1. Take 4 balls put them on one side take 4 more and put them on the other. If the weights are exactly the same, the odd ball out is the lightest.

#2. If one group weighed less, the odd ball out and the 4 that weighted more are discarded. Now split up the 4 from the lighter and weigh them 2 against 2. Discard the heavier group.

#3. you now have 2 balls left, weight them against eachother and the lighter is the correct ball.

 

[dullard]

Originally posted by: thraashman
Ok, the 9 balls riddle. I don't know if anyone posted this answer yet, doesn't look like it though I only skimmed through

I'll do this in weighings.

#1. Take 4 balls put them on one side take 4 more and put them on the other. If the weights are exactly the same, the odd ball out is the lightest...

You should have read more. The answer was posted and there is a discussion about how you are incorrect. What if the odd ball was heavy and not light?

 

[Strk]

Originally posted by: Born2bwire

Originally posted by: Garet Jax

Originally posted by: DaShen

Originally posted by: Atheus
Einstein's riddle. Apparently only 2% of the population can solve it.

1. In a street there are five houses, painted five different colours.
2. In each house lives a person of different nationality
3. These five homeowners each drink a different kind of beverage, smoke different brand of cigar and keep a different pet.

Hints:

1. The Brit lives in a red house.
2. The Swede keeps dogs as pets.
3. The Dane drinks tea.
4. The Green house is next to, and on the left of the White house.
5. The owner of the Green house drinks coffee.
6. The person who smokes Pall Mall rears birds.
7. The owner of the Yellow house smokes Dunhill.
8. The man living in the centre house drinks milk.
9. The Norwegian lives in the first house.
10. The man who smokes Blends lives next to the one who keeps cats.
11. The man who keeps horses lives next to the man who smokes Dunhill.
12. The man who smokes Blue Master drinks beer.
13. The German smokes Prince.
14. The Norwegian lives next to the blue house.
15. The man who smokes Blends has a neighbour who drinks water.

WHO OWNS THE FISH?

How is this only 2%?

All you have to do is write it down and build a matrix logic algorithm to figure out who is who. I see bird, dog, cat, and horse... so the last one would be fish. (it doesn't look that difficult).

I read through this quickly and it looks more like a trial and error problem that a logic problem.

A logic problem should be able to progress from start to finish with no missteps unless there was an error in the logic.

A trial and error problem is one where the only way to get to the right solution with no errors is through a miracle.

Trial and error problems suck....

I was able to do this without any trial and error. There is enough information in the problem to determine all the assignments without guessing.


EDIT: BigJ, oooh, that's another good way of doing it too.

Lame, it took me kind of long to finish that (about 20 minutes although I was technically watching TV while working on it), but I haven't done one of those in a long time. It really wasn't that hard though. It's not trial and error, it's all in how you set it up.

 

[crisscross]

Originally posted by: purbeast0
Here's one from an interview I had:
You have 3 guys in a room. No one can know the salary of what the other people makes, however when they all leave the room, they must know the average salary of the 3 people in the room. How do they do this?
(and remember, no one can know anyone elses salary).

Another one from an interview:
You have 2 fuses that each take 60 minutes to burn. There is no guarantee that they burn uniformly. It may take 59 minutes to burn 1% of the fuse, then 1 minute to burn the other 99% of the fuse. Or it could take 30 mins to burn 50% of the fuse, and 30 mins to burn the other 50% of the rope. There is no guarantee regardless.

Also you have no way to tell time or to tell when a specific amount of time has passed. You do not have a watch or clock with you, and you cannot just "count" out a certain amount of time.

How do you burn the fuses to guarantee they will burn in exactly 45 minutes.

My answer:

1st Person breaks up his salary into two unequal fractions & passes it on to 2nd & 3rd person respectively.

2nd Person adds his salary to the 1st person's fraction & passes it onto the third person.

3rd person adds his salary & 1st persons fraction to the no. received from 2nd person and divides it by 3 to get the average, which is shared with everyone.

Do you see any holes which I have missed?

 

[SoulAssassin]

Originally posted by: Garet Jax

Originally posted by: SoulAssassin
There is a lake that is perfectly square 30 feet on each side. In the middle of the lake is an island that is perfectly square 10 feet on each side. In the middle of the island is a castle w a damsel in distress who wants to bl0w you or something like that. You have two 2x4's that are 9.5 feet long each. Using the wood (the 2x4's that is) and only the wood how do you get on the island? Assume the water is infinitely deep and filled w sharks, republicans, terrorists, people w normal sounding first names spelled in stupid ways and other undesirables.

Couldn't you use the wood to pole vault across?

Realistically could you? Maybe. Probably not. We'll assume the sharks have frickin lasers.

 

[SoulAssassin]

Originally posted by: reverend boltron

Originally posted by: SoulAssassin
There is a lake that is perfectly square 30 feet on each side. In the middle of the lake is an island that is perfectly square 10 feet on each side. In the middle of the island is a castle w a damsel in distress who wants to bl0w you or something like that. You have two 2x4's that are 9.5 feet long each. Using the wood (the 2x4's that is) and only the wood how do you get on the island? Assume the water is infinitely deep and filled w sharks, republicans, terrorists, people w normal sounding first names spelled in stupid ways and other undesirables.

Hey, do you listen to Brian Regan?

Can't say that I do. I've heard of him but that's about it.

 

[dullard]

Originally posted by: Strk
It's not trial and error, it's all in how you set it up.

It has been years since I did that specific problem. But don't you need trial and error to realize that the first house is on the left and not on the right?

Or do you just use common sense that Einstein read from left to right, so that the house on the left is clearly the first house?

If you try the first house on the right, I think you reach a dead end quite quickly.

 

[allisolm]

Originally posted by: crisscross

Originally posted by: purbeast0
Here's one from an interview I had:
You have 3 guys in a room. No one can know the salary of what the other people makes, however when they all leave the room, they must know the average salary of the 3 people in the room. How do they do this?
(and remember, no one can know anyone elses salary).

My answer:

1st Person breaks up his salary into two unequal fractions & passes it on to 2nd & 3rd person respectively.

2nd Person adds his salary to the 1st person's fraction & passes it onto the third person.

3rd person adds his salary & 1st persons fraction to the no. received from 2nd person and divides it by 3 to get the average, which is shared with everyone.

Do you see any holes which I have missed?

Not seeing holes and it is more elegant than the other answers as it eliminates one step.

 

[Born2bwire]

Originally posted by: dullard

Originally posted by: Strk
It's not trial and error, it's all in how you set it up.

It has been years since I did that specific problem. But don't you need trial and error to realize that the first house is on the left and not on the right?

Or do you just use common sense that Einstein read from left to right, so that the house on the left is clearly the first house?

If you try the first house on the right, I think you reach a dead end quite quickly.

Well, I can not think of any real precedent in the English language where we would consider the numbering of a list as going right to left as opposed to left to right.

 

[Strk]

Originally posted by: dullard

Originally posted by: Strk
It's not trial and error, it's all in how you set it up.

It has been years since I did that specific problem. But don't you need trial and error to realize that the first house is on the left and not on the right?

Or do you just use common sense that Einstein read from left to right, so that the house on the left is clearly the first house?

If you try the first house on the right, I think you reach a dead end quite quickly.

I just went with the assumption he went with the typical 1, 2, 3, 4, 5. I suppose you could go the other way, but meh to that

 

[silverpig]

Originally posted by: allisolm

Originally posted by: crisscross

Originally posted by: purbeast0
Here's one from an interview I had:
You have 3 guys in a room. No one can know the salary of what the other people makes, however when they all leave the room, they must know the average salary of the 3 people in the room. How do they do this?
(and remember, no one can know anyone elses salary).

My answer:

1st Person breaks up his salary into two unequal fractions & passes it on to 2nd & 3rd person respectively.

2nd Person adds his salary to the 1st person's fraction & passes it onto the third person.

3rd person adds his salary & 1st persons fraction to the no. received from 2nd person and divides it by 3 to get the average, which is shared with everyone.

Do you see any holes which I have missed?

Not seeing holes and it is more elegant than the other answers as it eliminates one step.

Well that'll work, but there is a decent chance the guys could figure it out.

Say one guy has a salary of 50k and he breaks his up into 49999 and 1. It might be farily obvious to everyone what just happened, especially when the salaries to be added together are 129999 and 60001...

 

[Dritnul]

Originally posted by: Atheus
Einstein's riddle. Apparently only 2% of the population can solve it.

1. In a street there are five houses, painted five different colours.
2. In each house lives a person of different nationality
3. These five homeowners each drink a different kind of beverage, smoke different brand of cigar and keep a different pet.

Hints:

1. The Brit lives in a red house.
2. The Swede keeps dogs as pets.
3. The Dane drinks tea.
4. The Green house is next to, and on the left of the White house.
5. The owner of the Green house drinks coffee.
6. The person who smokes Pall Mall rears birds.
7. The owner of the Yellow house smokes Dunhill.
8. The man living in the centre house drinks milk.
9. The Norwegian lives in the first house.
10. The man who smokes Blends lives next to the one who keeps cats.
11. The man who keeps horses lives next to the man who smokes Dunhill.
12. The man who smokes Blue Master drinks beer.
13. The German smokes Prince.
14. The Norwegian lives next to the blue house.
15. The man who smokes Blends has a neighbour who drinks water.

WHO OWNS THE FISH?

got it after ~10-15mins



edit: we aint posting this answer oops

 

[jbkane26]

At a movie theater, the manager announces that they will give a free ticket to the first person in line whose birthday is the same as someone who has already bought a ticket. You have the option of getting in line at any time. Assuming that you don't know anyone else's birthday, that birthdays are distributed randomly throughout the year, etc., what position in line gives you the greatest chance of being the first duplicate birthday?

 

[jbkane26]

Decapitate me and all becomes equal. Then truncate me and I become second. Cut me front and back and I become two less than I started.

What am I?

 

[elkinm]

Originally posted by: purbeast0

Originally posted by: Garet Jax

Originally posted by: purbeast0
Here's one from an interview I had:
You have 3 guys in a room. No one can know the salary of what the other people makes, however when they all leave the room, they must know the average salary of the 3 people in the room. How do they do this?
(and remember, no one can know anyone elses salary).

Another one from an interview:
You have 2 fuses that each take 60 minutes to burn. There is no guarantee that they burn uniformly. It may take 59 minutes to burn 1% of the fuse, then 1 minute to burn the other 99% of the fuse. Or it could take 30 mins to burn 50% of the fuse, and 30 mins to burn the other 50% of the rope. There is no guarantee regardless.

How do you burn the fuses to guarantee they will burn in exactly 45 minutes.

1st)

Have the first guy choose some random number and then add his salary to it. Write it down. Pass the paper to the second guy who adds his salry to the number on the paper and writes it on a new piece. The third guy adds his salary to the number on the latest paper and then writes it down. He passes the paper to the first guy who subtracts the random number he added to start. Divides by 3.

2nd)

I will have to think a little more about it.

ding ding with the 1st one! that was exactly what I said in the interview, however they then gave me a much simpler answer. Everyone just adds an arbitrary # to their salary. They then add these 3 numbers up. They then each subtract their 3 numbers from the total and divide it by three.

It was basically about encryption.

And it's very similar to what these other guys were saying too, but it was just a little off.

Close, but it is still wrong because if they subtract the arbitrary number then the others know the number they subtracted and then know the solution. It cannot work if you can know what each guy subtracted.
So you can write three arbitrary numbers on three papers, take one and add that to your salary, then mix them again, add the three numbers together and and subtract from the combined salaries + numbers, but unless the papers were identical you can still guess.

Now for the true solution. You agree on some arbitrary number, say 150 and split it into three non exact numbers, say 100, 20 and 30, put in a random pile. Each one takes one and adds it to their salary. They add the combined numbers, subtract 150 and divide by three.

They might still know the papers each got but with 150 nobody will explicitly know what each guy added, just that a total of 150 was added.


Also crisscross has the best answer as with his there truly is no way for anyone to guess what the other numbers are, without sharing some more information with each other.

I really don't like problems like this, where the interviewer does not even know the correct answer and wording is a huge concern.

 

[elkinm]

My favorite problem, probably quite famous.

You need to get to some town, but are at a fork with two roads to choose from.

You know there are two villages at the crossings but you don't know which one is witch.

Every person from one village will always tell the truth, while the other village everyone will always lie, but you don't know who is from which village.
And every person in both villages knows which one is the next path.

You can only ask one person, and only one question about the crossing.
EDIT: Also, all the villagers can only reply with "yes" or "no" answers.

What one question can you ask to find out which road to take.



Also, and addition to the fuses, how can you take only one of those 60 min fuses and have it burn for exactly 15 minutes.
Warning, the solution is mathematically sound but might not be realistically feasible.

Good Luck

 

[madworm]

Originally posted by: elkinm
My favorite problem, probably quite famous.

You need to get to some town, but are at a fork with two roads to choose from.

You know there are two villages at the crossings but you don't know which one is witch.

Every person from one village will always tell the truth, while the other village everyone will always lie, but you don't know who is from which village.
And every person in both villages knows which one is the next path.

You can only ask one person, and only one question about the crossing.

What one question can you ask to find out which road to take.



Also, and addition to the fuses, how can you take only one of those 60 min fuses and have it burn for exactly 15 minutes.
Warning, the solution is mathematically sound but might not be realistically feasible.

Good Luck

you will ask the villager. which direction would the villagers in the other village say "you" will point me to?

then you just do the opposite...

 

[elkinm]

Originally posted by: madworm

Originally posted by: elkinm
My favorite problem, probably quite famous.

You need to get to some town, but are at a fork with two roads to choose from.

You know there are two villages at the crossings but you don't know which one is witch.

Every person from one village will always tell the truth, while the other village everyone will always lie, but you don't know who is from which village.
And every person in both villages knows which one is the next path.

You can only ask one person, and only one question about the crossing.

What one question can you ask to find out which road to take.



Also, and addition to the fuses, how can you take only one of those 60 min fuses and have it burn for exactly 15 minutes.
Warning, the solution is mathematically sound but might not be realistically feasible.

Good Luck

you will ask the villager. which direction would the villagers in the other village say "you" will point me to?

then you just do the opposite...

That would work. Just to rephrase the question a little. If I were to ask someone from the other village which road leads to town, which way would they point me. Just to make it complete but it works.

The problem should have one more part to it. The villagers can only say yes or no to strangers like you. Sorry I forgot about that part. But it is simple enough that your statement is good, just needs to be more specific to lead to a yes or no answer.
I will add that part to my original problem.