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# Random Riddles

• 16-07-2007 8:59am
Registered Users Posts: 550 ✭✭✭

Came across these before. I know that some of them have appeared on this forum previously in different guises and most a quite easy but I thought I'd post the lot for newbies.

Bulbs
Imagine you are in a room with 3 switches. In an adjacent room there are 3 bulbs (all are off at the moment), each switch belongs to one bulb. It is impossible to see from one room to another. How can you find out which switch belongs to which bulb, if you may enter the room with the bulbs only once?

Ball in a Hole
A table tennis ball fell into a tight deep pipe. The pipe was only a bit wider then the ball, so you can not use your hand. How would you take it out, with no damage?

The Man in the Elevator
A man lives on the tenth floor of a building. Every morning he takes the elevator down to the lobby and leaves the building. In the evening, he gets into the elevator, and, if there is someone else in the elevator - or if it was raining that day - he goes back to his floor directly. Otherwise, he goes to the seventh floor and walks up three flights of stairs to his apartment. Can you explain why?
(This is probably the best known and most celebrated of all lateral thinking logic puzzles. It is a true classic. Although there are many possible solutions which fit the initial conditions, only the canonical answer is truly satisfying.)

Ball
How can you throw a ball as hard as you can and have it come back to you, even if it doesn't hit anything, there is nothing attached to it, and no one else catches or throws it?

Magnet
This is a logic puzzle published in Martin Gardner's column in the Scientific American.
You are in a room where there are no metal objects except for two iron rods. Only one of them is a magnet.
How can you identify this magnet?

Castle
A square medieval castle on a square island was under siege. All around the island, there was a 10 metre wide water moat. But the conquerors could make foot-bridges only 9.5 metres long. Nevertheless a wise man was able to figure out how to get over the water. What do you think was his advice?
(There's a place on the other side to put the bridge against, not just a sheer wall. the water moat has square corners - that section of the moat is about 14.1 metres wide.)

Biology
Let's say some primitive organisms divide themselves every minute in two equal parts which also divide the next minute and so on. The saucer in which we started observing this process was full at 12.00.
When was it half full?

Sheikh's Heritage
An Arab sheikh tells his two sons to race their camels to a distant city to see who will inherit his fortune. The one whose camel is slower will win. The brothers, after wandering aimlessly for days, ask a wise man for advice. After hearing the advice they jump on the camels and race as fast as they can to the city.
What does the wise man say?

Philosopher's Clock
This is an old logic puzzle. One philosopher had a old heavy clock in his house, which he had forgotten to wind up. He had no other clock, watch, radio, TV, phone or any other device telling the time. So when his clock stopped he went to a friend, stayed there the whole night and when he came home, he knew the right time.
How could he know?

Masters of Logic Puzzles I
Three masters of logic wanted to find out who was the wisest one. So they invited the grand master, who took them into a dark room and said: "I will paint each one of you a red or a blue dot on your forehead. When you walk out and you see at least one red point, raise your hands. The one who says what colour is the dot on his own forehead first, wins." Then he painted only red dots on every one. When they went out everybody had their hands up and after a while one of them said: "I have a red dot on my head."
How could he be so sure?
Solution

Masters of Logic Puzzles II
Try this. The grand master takes a set of 8 stamps, 4 red and 4 green, known to the logicians, and loosely affixes two to the forehead of each logician so that each logician can see all the other stamps except those 2 in the moderator's pocket and the two on her own head. He asks them in turn if they know the colors of their own stamps:
A: "No."
B: "No."
C: "No."
A: "No."
B: "Yes."
What are the colors of her stamps, and what is the situation?

Enjoy!

• Registered Users Posts: 5,962 ✭✭✭

Oooh very nice

Lets see if I can get some of them

Ball
Throw the ball in the air

Biology
11:59

The man in the elevator
I heard this a few times, the man is too small to reach the top floor button. When there's someone in the lift with him he asks them to press it for him.

Ball in the hole
It doesn't say if there's only access to one end of the pipe or how long it is, so it might be possible to poke the ball out from one end with a stick.

Sheikh's Heritage
Heard this one before too, they swap horses

Philosopher's Clock
He brings his clock with him and sets it to his friends clock

Obviously I'm not much good at these so I must be a 'newbie' lol

• Registered Users Posts: 950 ✭✭✭

Ball in a hole
Pee into the pipe until the ball floats to the top, then take it out

Castle
Like This:

• Registered Users Posts: 550 ✭✭✭

Yup, except I probably should have been clearer with the Clock one. When I said the clock was heavy, I meant that it couldn't be lifted out of the house.

• Registered Users Posts: 1,837 ✭✭✭

Teg Veece wrote:
Masters of Logic Puzzles I
Three masters of logic wanted to find out who was the wisest one. So they invited the grand master, who took them into a dark room and said: "I will paint each one of you a red or a blue dot on your forehead. When you walk out and you see at least one red point, raise your hands. The one who says what colour is the dot on his own forehead first, wins." Then he painted only red dots on every one. When they went out everybody had their hands up and after a while one of them said: "I have a red dot on my head."
How could he be so sure?
Solution

Watch me screw up the explanation of this;

Let's say the three people are A (the guy who get's it right), B and C

A can see B & C with a red dot each, and both with hands up.

Now If A thinks about what B sees, he realises that B must see C with a red dot, and A with a red/blue dot.

Let's suppose A has a blue dot. Thus B sees A with blue and C with red. B also sees both A and C have hands up. B would thus realise that C sees A with a blue dot and B with a red dot (as C has his hand up) and B would realise he himself has a red dot.

Back to the point of ciew of A, A realises that because B (or, by implication, C) has not declared that they have a red dot, the scenario above where A has a blue dot is impossible, and A must have a red dot.

• Registered Users Posts: 689 ✭✭✭

Teg Veece wrote:
Yup, except I probably should have been clearer with the Clock one. When I said the clock was heavy, I meant that it couldn't be lifted out of the house.

not a solution but hidden anyway
I thought that answer might have been right as well... and the reason he stayed the night was to recover after the exertion of carring the clock all the way.

• Registered Users Posts: 689 ✭✭✭

Bulbs
This is well known...
Turn on one switch and leave on for 10 minutes, then turn off and switch another switch on. Then go into the room... the bulb on corresponds to the switch that is on. Feel both bulbs that are off, the one that is warm corresponds to the one that was on for 10 minutes.

The Man in the Elevator
I got this one myself, was very happy as I considered it difficult.
As above... but the reason he goes to his own floor when it's raining is that he is carrying an umbrella which he can use to hit the button...

Magnet
This one I am still working on...

Sheikh's Heritage
As above.. except that they swap camels not horses.... lol...

Philosopher's Clock
I am also still working on this one, I had a partial solution but it's wrong apparently.

Masters of Logic Puzzles II
This one is tough, answer shortly...

Cheers
Joe

• Registered Users Posts: 689 ✭✭✭

Teg Veece wrote:
Masters of Logic Puzzles II
Try this. The grand master takes a set of 8 stamps, 4 red and 4 green, known to the logicians, and loosely affixes two to the forehead of each logician so that each logician can see all the other stamps except those 2 in the moderator's pocket and the two on her own head. He asks them in turn if they know the colors of their own stamps:
A: "No."
B: "No."
C: "No."
A: "No."
B: "Yes."
What are the colors of her stamps, and what is the situation?

AHA!!! I have it!
A has two of one colour, C has two of the other colour, B has one of each colour, and there is one of each colour in the pocket.

So A sees
B - one of each
C - two red (say)
A can't tell... he could either have two green or one of each...

B sees
A - two green
C - two red
B can't tell.... (but he does know he doesn't have two red as A would see 4 red and know he had two green... and A didn't know.... but B could still have two green or one of each so he must answer 'No')

C sees
A - two green
B - one of each
C can't tell... (but he does know he doesn't have two green as B would see 4 green and know he had two red, he could either have two red (as he does have) or he could have one of each, in either case neither B nor A would know the answer)

A sees..
no new info, can't tell

B however now knows that he doesn't have two green.. because if he had two green then C would see A - two green, B - two green and C would know that he has two red... likewise he doesn't have two red as A would have known from the start that A had two green... but A and C don't know so B must have one of each...

This is the only situation I can think of that satisfies the conditions...

Great puzzle, quite tough...

• Registered Users Posts: 689 ✭✭✭

Teg Veece wrote:
Philosopher's Clock
This is an old logic puzzle. One philosopher had a old heavy clock in his house, which he had forgotten to wind up. He had no other clock, watch, radio, TV, phone or any other device telling the time. So when his clock stopped he went to a friend, stayed there the whole night and when he came home, he knew the right time.
How could he know?
Hmmmm... I'm not sure if this is correct... but he could borrow a portable clock from his friend and use that to set his own clock when he gets home... he could also borrow an hour glass, note the time, turn over the hour glass, head home carrying the hour glass and set his clock to the time he noted plus one hour when the hour glass ends...

• Registered Users Posts: 689 ✭✭✭

Teg Veece wrote:
Magnet
This is a logic puzzle published in Martin Gardner's column in the Scientific American.
You are in a room where there are no metal objects except for two iron rods. Only one of them is a magnet.
How can you identify this magnet?

This one is causing me some problems... I have a few ideas but I think they're wrong.
1) Cut both bars in half.. (with a piece of chees perhaps, lol).
This would make it easy to determine the magnet as two of the pieces from the non magnet are simply not magnets and don't stick.

2) Maybe if they are held in a T shape (by the horizontal piece) only the magnet will 'stick' as the vertical piece, if they are reversed they won't 'stick'.... (possibly they will only stick if the magnet is the horizontal piece... or maybe they have to be held in an upside down T... in fact I think this may be it... the magnet must be the vertical piece if they are to stick in an upside down T, could be wrong though obviously)

3) Look for the word 'Magnet' on one of the bars, or a red marking on the end... I'm clutching at straws now....

• Registered Users Posts: 1,555 ✭✭✭

Teg Veece wrote:
Magnet
This is a logic puzzle published in Martin Gardner's column in the Scientific American.
You are in a room where there are no metal objects except for two iron rods. Only one of them is a magnet.
How can you identify this magnet?

This is how I'd do it:
Get a piece of string and hang one bar, by its centre, horizontally from the ceiling. If it rotates to face north-south, then its the magnet.

• Registered Users Posts: 689 ✭✭✭

But we don't know which way is N-S, but I know what you mean, only one of the rods will tend to move at all and it will always tend to return to the same position... I had thought of making a home made compass, by resting the rods on something floating in water but I wasn't sure if we'd have those things...

What do you think about the upside down T shape, holding the vertical piece... the field lines are strongest at the ends and weakest in the middle of the rods so I think they may only 'stick' in this configuration if the magnet is the vertical piece... but it does depend on the magnets strength relative to the weight of the rods, I feel if these were chosen correctly the upside down T shape would work... possible it would work for most combinations of weights and strengths?

• Registered Users Posts: 1,555 ✭✭✭

My answer does of course depend on the weight/size of the bars as well.

• Registered Users Posts: 550 ✭✭✭

I think you'd have to accept both the N-S and T-shape solutions for the magnets question.

With regards the philospher's clock question; there is an assumption made that the path from the phiospher's house to his friends house is completely flat. Hope that helps!

• Registered Users Posts: 6,592 ✭✭✭

Teg Veece wrote:
With regards the philospher's clock question; there is an assumption made that the path from the phiospher's house to his friends house is completely flat. Hope that helps!
Ah! Would it be anything along the lines of:
He sets the clock to 12. Walks to his friends house and back. Notes the amount of time that has passed. Halves it. Then walks back to his friends house. And adds that amount onto the time that it was when he leaves his friends house the day after?

• Registered Users Posts: 550 ✭✭✭

Ro: maaan! wrote:
Ah! Would it be anything along the lines of:
He sets the clock to 12. Walks to his friends house and back. Notes the amount of time that has passed. Halves it. Then walks back to his friends house. And adds that amount onto the time that it was when he leaves his friends house the day after?

You almost have it! But he only needs to go to his friend's house once...

• Registered Users Posts: 689 ✭✭✭

OK, philosophers clock...

I think my previous solution is workable but I now have another... this is posted before reading the answers above, I will read them after...
Well, if we suppose he can walk at a steady rate, then the time taken to get to his friends house is equal to the time taken to return...

So he can wind his clock at home, note the time... walk to his friends house and immediately note the time... then he can relax. The next morning he notes the time, works out how for how long he was at his mates gaf.. then returns home (after noting the time of departure). (Time of departure - Time of arrival = time spent doing whatever..)

He can now look at his own clock, subtract the time he was buzzing with his mate i.e not walking (from the total time elasped as shown on his own clock), ... then divide the remaining time by two, and add this to the time of departure... this should be the current time...

This would be less accurate than using a hourglass or a portable clock but it should be reasonably close, when everybody is using wind up clocks nobody is going to know the time to the second anyway..

edited to clarify... added line (from the total time elasped as shown on his own clock)

Cheers
Joe

• Registered Users Posts: 6,374 ✭✭✭

If you rub an iron rod along a magnet, it will become magnetised temporarily.
Watch which poles attract and repel. between the two.

Do this again on the other rod, to reverse the magnetism on the pole (or run the process of elimination).
If it still reverses, you have an iron rod.
If it stays the same polarity it was the original magnet.

• Registered Users Posts: 4,849 ✭✭✭

Bulbs;
by using the heat of the bulbs(heard before)

Balls;
fill the pipe with water

Man in elevator
he's a dwarf (heard before)

Ball
throw it up in the air

Magnet and castle, I'm still working on

Biology
you say they divide into two equal halves. That means the bowl never loses any matter

Sheikh
swap camels (heard before)

Masters of Logic 1
B realises that A would have realised that if A had a blue dot, C would have his hand down and would therefore know that he himself had a blue dot. So, since A hasn't said anything, B must also have a red dot to disallow that to take place

And still working on the last one.

Thanks, some good ones in there

• Closed Accounts Posts: 950 ✭✭✭

Masters of Logic Puzzles I
If there were two blue and one red, the red one would see 2x blue and immediately know he was red.

But no-one says anything. So it isn't two blue, one red.

If there were two red and one blue, then either red would see blue+red and know that he must be red.

But no-one says anything. So it isn't two red, one blue.

So all three are red. The first one to realise what the prolonged silence means knows he must be red.

(sorry if I haven't explained it well)

• Registered Users Posts: 393 ✭✭

This might seem stupid but I am new on here - how do I see responses and remove spoliers or is it possible ?