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You say gold.

Either

Gold/Gold face up
Gold/Gold face down
or
Gold/Siver face up.

So there is a two in three chance of it being the gold gold coin.

This is most closely related to the twin "paradox".

Sounds also very much like the Monte Hall problem to me, but you're right... the switching option is missing.

They also had the Monte Hall problem in one episode of Num3ers, and the geek with the curls also forgot to mention the switching option.

BTW, if you want to read a really good explanation on the solution of Monte Hall, read Derren's book "Tricks of the Mind". He's delivering two really good explanations for the phenomenon... and both of them only work when you have to choice to switch.

Or I am completely wrong, too...???

Mr Grue,

sound good, but isn't there also a siver/silver face up and a silver/silver face down option that we forget here all the time?

Sort of.

The choices are not

GG GS SG SS

There are actually six options in total, and three where gold can be uppermost:

GG GG SG GS SS SS

(The double sided coin has to be considered twice as it skews the random drawing - there is, after all a 4/6 chance of drawing a coin with the same top and bottom colour)

GG GG GS

This means that there is a 2/3 chance of predicting the underside of the coin, if you choose the same colour as the topside.

The relationship to the Monte Hall problem is simply the prior information skewing the probabilities. If the coin was unseen the chances would still be 3/6.

The original Monte Hall paradox was to have three doors, A B and C. The audience member would choose one of these (say "A") and then be shown a losing door (say "C")

They would be given the option to stay with A or switch.

If they switch to B, they are guarenteed a 1/2 chance of choosing the correct door (as C has been eliminated), and if they stick with A they have their original 1/3 choice.

Technically the coins from the bag is like this, but in reverse.

Dominic - you are correct .

Basically, whatever colour is facing up - that's the colour that you guess will be the underside colour. Probability says that you'll win 2 out of 3 times.
The only time you lose is if the gold/silver token is on the table.
If the gold/gold is on the table then you win 'cos you would have guessed that gold is the underside and if silver/silver is on the table then you win cos you would have guessed that silver is the underside.

I'm sure some people are gonna disagree with this - but feel free to fire away ......

Anyway, your riddle next Dominic .

I'm going to agree so much I'll do it ahead of time.

Yeah, I was afraid of that

Something with a bit of a Conan Doyle vibe to it:


Last night a man found out that his brother had been sleeping with his wife. Fuelled by brandy, opiates and rage, their argument turned to blows, and in the heat of the moment the man drew a revolver and shot his brother in the face.

Stumbling drunk, panicked and confused, the man scrambled for his study, and bolted the door behind him. The maid, attracted by the gun shot and the screams ran to the door just in tine to hear him scream “No! It can’t be! It isn’t possible!” followed by another gunshot, then silence.

When the police battered down the door, they found the man dead on the floor in a pool of blood, there was a look of abject terror on his face, and he had clearly shot himself in the head.

Scotland Yard were happy with the deduction that he had killed himself in guilt, but there was no suicide note, and the strange exclamation that the maid overheard was enough for the family to contact a consulting detective agency.

The great detective swiftly deduced that, although the cause of death was obvious and ordinary, the vision that caused him to shoot himself was definitely extra-ordinary.

What did the man see that drove him to suicide?

His wife had a twin sister he didn't know about before... so the other guy did NOT sleep with his wife?

Nope, and that wouldn't explain what he saw.

There's a touch of the literary license here - say, Jonathan Creek or Holmes, so the answer may not be entirely credible, but definitely rational.

He didn't shoot his brother, he shot someone else thinking it was his brother due to being drunk?

Nope, he definitely shot his brother - both Scotland Yard and the Great Detective would have confirmed that.

He meant to shoot his brother with another persons gun but used his own by mistake? After looking at the gun he realised his mistake

Alternative theory is that he has a set of twin brothers(or he is one of triplets) and accidentally shot the wrong twin

Nope. Same gun used in both shootings

The question is what did he see, alone in his locked study, that terrified him to the point of suicide?

Dominic Rougier wrote:Nope. Same gun used in both shootings

The question is what did he see, alone in his locked study, that terrified him to the point of suicide?

I meant only one gun was used. What I was getting at was he planned to use another gun (for instance a business rivals) gun in order to incriminate him, but accidentally fired the wrong one. After going into his study, he looked at the gun, realized his mistake and figured that he may as well use it to kill himself because hes going to be executed for murder anyway

Ah! possibly... that didn't occur to me.

That would be a solution, but I was going for something a touch more melodramatic...