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

[Garet Jax]

You have nine balls. 8 are physically identical in weight and dimensions. 1 is the identical dimension as the others, but is a different weight.

You have an equal arm balance and are allowed 3 weighings. How do you determine the 1 ball that is different than the other 8 and whether it is heavier or lighter than the other 8.

 

[pontifex]

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?

there's a few problems with that ....1 - its a 2x4 and 2 - Assume the water is infinitely deep

 

[pontifex]

Originally posted by: Garet Jax
You have nine balls. 8 are physically identical in weight and dimensions. 1 is the identical dimension as the others, but is a different weight.

You have an equal arm balance and are allowed 3 weighings. How do you determine the 1 ball that is different than the other 8 and whether it is heavier or lighter than the other 8.

pick each one up until you find one thats heavier than the others. don't even need to use the balance.

 

[Stuxnet]

Originally posted by: her209
A man and his son are driving down the highway. They get into a car wreck. Two ambulances come and take each of them to two different hospitals. The son goes into the operating room and the doctor takes one look and says "I can't operate on this boy, he's my son."

How is it possible that anyone even considers this a riddle?

Fixed

 

[Garet Jax]

Originally posted by: pontifex

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?

there's a few problems with that ....1 - its a 2x4 and 2 - Assume the water is infinitely deep

Right but you only have 10 feet to cross. If you put your hands on the top of the 2X4 and manage to get the bottom right at the edge of the land, then your body can easily make up the remaining 6 inches.

 

[Garet Jax]

Originally posted by: pontifex

Originally posted by: Garet Jax
You have nine balls. 8 are physically identical in weight and dimensions. 1 is the identical dimension as the others, but is a different weight.

You have an equal arm balance and are allowed 3 weighings. How do you determine the 1 ball that is different than the other 8 and whether it is heavier or lighter than the other 8.

pick each one up until you find one thats heavier than the others. don't even need to use the balance.

Difference in weight is indistinguishable by a human and you are only allowed 3 weighings.

 

[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?

 

[dullard]

Misread.

 

[Born2bwire]

Originally posted by: RapidSnail

Originally posted by: chuckywang
You ALWAYS switch.

Explain your answer, please.

.... Please don't turn into another plane on a conveyor belt situation...

 

[allisolm]

That'll give you a pile of 4 unknown balls and another pile of 4 balls known to be the same weight.

but there are 9 balls.

 

[dullard]

Originally posted by: allisolm
That'll give you a pile of 4 unknown balls and another pile of 4 balls known to be the same weight.

but there are 9 balls.

Oops. I misread it as 8 balls. Let me retype it. Fixed:

Originally posted by: Garet Jax
You have nine balls. 8 are physically identical in weight and dimensions. 1 is the identical dimension as the others, but is a different weight.

1) Weigh any 3 unknowns vs any 3 unknowns.
1a) If they are equal, you?ll have 3 unknowns and 6 equals.
1b) If they are unequal, you?ll have 3 light, 3 heavy, and 3 equals. Skip to step #4

2) Weigh the 3 unknowns vs any 3 equals.
2a) If the 3 unknowns are light, you know the odd ball is light.
2b) If the 3 unknowns are heavy, you know the odd ball is heavy.

3) Thus take any one of the unknown balls vs another unknown ball. If these are equal, the ball you didn't weigh was the odd ball. If those two are unequal, the one that matches the heavy or light side is the odd ball. Problem solved.

4) Weigh 3 light vs 3 equals.
4a) If they are equal, you know the odd ball is heavy. You'll also have a pile of 6 equals and a pile of 3 unknowns. Go to step #3.
4b) If they are unequal, you know the odd ball is light. You'll also have a pile of 6 equals and a pile of 3 unknowns. Go to step #3.

There are much more difficult versions of this problem. I'm glad you gave an easy to type answer version.

 

[iwantanewcomputer]

***SPOILER ALERT***

Originally posted by: Garet Jax
You have nine balls. 8 are physically identical in weight and dimensions. 1 is the identical dimension as the others, but is a different weight.

You have an equal arm balance and are allowed 3 weighings. How do you determine the 1 ball that is different than the other 8 and whether it is heavier or lighter than the other 8.

this can be done in 2 weighings right?










Dullard...i believe that is too complicated. I thought to weigh 3v3...if they are unequal, you know which group of 3 has the heavy one, if they are equal, the group you didnt weigh has it.

now from the heavy group, weigh any two. the heavy one will be the lower one on the balance, or the one sitting out if they are equal.

 

[Garet Jax]

Originally posted by: iwantanewcomputer

Originally posted by: Garet Jax
You have nine balls. 8 are physically identical in weight and dimensions. 1 is the identical dimension as the others, but is a different weight.

You have an equal arm balance and are allowed 3 weighings. How do you determine the 1 ball that is different than the other 8 and whether it is heavier or lighter than the other 8.

this can be done in 2 weighings right?

I don't think so since the control group is not the same size as the weighing groups.

 

[allisolm]

I thought to weigh 3v3...if they are unequal, you know which group of 3 has the heavy one, if they are equal, the group you didnt weigh has it.

now from the heavy group, weigh any two. the heavy one will be the lower one on the balance, or the one sitting out if they are equal.

but the one ball could be lighter, not heavier.

 

[dullard]

Originally posted by: iwantanewcomputer
Dullard...i believe that is too complicated. I thought to weigh 3v3...if they are unequal, you know which group of 3 has the heavy one, if they are equal, the group you didnt weigh has it.

now from the heavy group, weigh any two. the heavy one will be the lower one on the balance, or the one sitting out if they are equal.

Let me try your logic. Tell me if I understand your post correctly.

On your first weighing, you take any 3 random balls and weigh it vs any 3 random balls. Suppose they are unequal. How do you know if the odd ball is heavy or light? You immediately jumped to saying you know it is heavy. But how do you know that?

 

[Garet Jax]

Originally posted by: iwantanewcomputer
***SPOILER ALERT***

Originally posted by: Garet Jax
You have nine balls. 8 are physically identical in weight and dimensions. 1 is the identical dimension as the others, but is a different weight.

You have an equal arm balance and are allowed 3 weighings. How do you determine the 1 ball that is different than the other 8 and whether it is heavier or lighter than the other 8.

this can be done in 2 weighings right?

If you know that one is heavier or lighter, then it can be done in two weighings, but since you don't, I don't believe it can be done.








Dullard...i believe that is too complicated. I thought to weigh 3v3...if they are unequal, you know which group of 3 has the heavy one, if they are equal, the group you didnt weigh has it.

now from the heavy group, weigh any two. the heavy one will be the lower one on the balance, or the one sitting out if they are equal.

 

[Born2bwire]

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?

*SPOILER ALERT*


















The German has the fish.

 

[DaShen]

Originally posted by: RapidSnail
Probably familiar, but here goes.

We assume a gameshow situation with a host and a contestant. There are three doors. One door holds a prize, while the other two doors contain goats. The contestant starts off by choosing one door out of the three. After he has picked, the host proceeds to open one of the doors which has a goat (Note that this action is required of the host whether or not the contestant picked the right door). The host then offers the contestant the option of switching to the other door or remaining with his initial choice.

What is the chance of the contestant winning the prize by switching doors?


By the way, I am guessing that this brain-teaser is fairly well known, so I must ask that any answers include a spoiler alert at the top of the post. Please do this for other riddles/brain-teasers as well.

This is a probability question, not a brain teaser.

The probability of winning is better if you switch.

 

[iwantanewcomputer]

Originally posted by: dullard

Originally posted by: iwantanewcomputer
Dullard...i believe that is too complicated. I thought to weigh 3v3...if they are unequal, you know which group of 3 has the heavy one, if they are equal, the group you didnt weigh has it.

now from the heavy group, weigh any two. the heavy one will be the lower one on the balance, or the one sitting out if they are equal.

Let me try your logic. Tell me if I understand your post correctly.

On your first weighing, you take any 3 random balls and weigh it vs any 3 random balls. Suppose they are unequal. How do you know if the odd ball is heavy or light? You immediately jumped to saying you know it is heavy. But how do you know that?

oh, i assumed we knew 1 ball was heavier than the rest...that's the way i originally heard it

 

[dullard]

Originally posted by: iwantanewcomputer
oh, i assumed we knew 1 ball was heavier than the rest...that's the way i originally heard it

No problem. I misread it the first time I posted too. If you look above, I misread it as being 8 total balls and allisolm corrected me.

This problem has so many variations. The key to virtually off of them is to get it into groups of 3, because you then get the most information with the least number of weighings. I may be overly complicated for this specific problem, but I just am using the general rule of getting them into groups of 3 so that people who try this with different numbers of balls and different numbers of weighings will hopefully use the 3 ball technique.

 

[loic2003]

So there's a jet-powered plane which is sitting on a conveyor belt...

 

[DaShen]

Originally posted by: dopcombo
Er, you nail the wooden planks in a T shape and place the structure at the corner of the lake?

NICE :thumbsup:

 

[BigJ]

Originally posted by: DaShen

Originally posted by: dopcombo
Er, you nail the wooden planks in a T shape and place the structure at the corner of the lake?

NICE :thumbsup:

If you're going to the Hardware store to get a hammer and nails, you might as well pick up a big ass ladder too

 

[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).

 

[BigJ]

Originally posted by: DaShen

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).

Most of the population can't even add or subtract without a calculator, and you expect them to do this?