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[tubaman]

I'd say 1/2

[kitz]

From Maths is fun

Our Solution:
2/3

You know that you do not have Bag B (two black marbles) so there are three possibilities

You chose Bag A, first white marble. The other marble will be white
You chose Bag A, second white marble. The other marble will be white
You chose Bag C, the white marble. The other marble will be black

So 2 out of 3 possibilities are white.

[sevenlayermuddle]

My reasoning was sound, then

This might be lowering the intellectual tone a little, but it’s my favourite numbers riddle….

Why does 6 fear 7?

[tubaman]

From Maths is fun

Our Solution:
2/3

You know that you do not have Bag B (two black marbles) so there are three possibilities

You chose Bag A, first white marble. The other marble will be white
You chose Bag A, second white marble. The other marble will be white
You chose Bag C, the white marble. The other marble will be black

So 2 out of 3 possibilities are white.

Hmm, I suppose in a strict mathematical sense this is correct but the question was "What is the probability that the remaining marble from the same bag is also white?", and as the first two options above are effectively identical that's why I went for 1/2.   

[roseway]

Hmm, I suppose in a strict mathematical sense this is correct but the question was "What is the probability that the remaining marble from the same bag is also white?", and as the first two options above are effectively identical that's why I went for 1/2.   

The two white marbles in bag A are physically separate. It might help to think of them as W1 and W2. There are two ways in which a white marble can be taken from bag A, either by taking W1 or by taking W2.

[tubaman]

The two white marbles in bag A are physically separate. It might help to think of them as W1 and W2. There are two ways in which a white marble can be taken from bag A, either by taking W1 or by taking W2.

My logic was thus:
Bag B is discounted as it only had black marbles.
If we had chosen from Bag A then we know that the other marble is white hence the answer to the question would be 1.
If we had chosen Bag C then we know the other marble is black hence then answer to the question would be 0.
(1+0)/2 = 1/2

 

[sevenlayermuddle]

My logic was thus:
Bag B is discounted as it only had black marbles.
If we had chosen from Bag A then we know that the other marble is white hence the answer to the question would be 1.
If we had chosen Bag C then we know the other marble is black hence then answer to the question would be 0.
(1+0)/2 = 1/2

 

But… since you already found a white marble, you are already more likely to be holding bag A than C.

[tubaman]

But… since you already found a white marble, you are already more likely to be holding bag A than C.

At this point I think my tiny mind might be about to explode!
 

Perhaps my logic works if the question had been "If you picked the other marble from the same bag what is the probability of it being white"?

[4candles]

At this point I think my tiny mind might be about to explode!
 

You are not alone! 

[sevenlayermuddle]

You are not alone! 

Just to emphasise, rest assured that my own contribution (reply #32) is at a very different extreme of mathematical complexity.   That said, anybody who’s not previously aware of it, yet ‘calculates’ the correct answer, earns my sincere respect.   

[kitz]

Some puzzles may have more than one answer depending on how you look at things or how you interpret the wording which can take you down a diifferent path.

I'm in the 2/3 camp, but just ended up tying myself in knots too when trying to explain it

[kitz]

This might be lowering the intellectual tone a little, but it’s my favourite numbers riddle….

Why does 6 fear 7?

Because seven, eight, nine. 

[sevenlayermuddle]

Because seven, eight, nine. 

Correct, of course.

[craigski]

Maybe 7 was a 6 offender?

[tubaman]

Maybe 7 was a 6 offender?

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