Hi MathsIsFun,

I've tried this today and got just one solution:  the one given on the solution page.

I wonder if  jr.williams was interpreting the information the same way as me.

I assumed that the four picked their marbles in the order given; looking at their card but without revealing it to the others; and then making their declaration without revealing what they thought the third marble was to the others.  Only then did the next one in order pick their marbles.  Thus, each is able to use information from what has previously been revealed.

Bob

hi phrontister

I started like this.

Tom draws two BBs  I'll use small letters to indicate what label possibilities there may be.

So consider all four labels:

BB  label bby
BB  label bbb
BB  label byy
BB  label yyy

He knows he must have either BBB or BBY.  If the labels are byy or yyy he cannot tell his last marble.

But he says he can tell.

So there are only two possibilities:

BB label bby ..... he deduces the third marble is B
BB label bbb ....  he deduces the third marble is Y.

Then I did a similar analysis for D... without taking the above into account.  Again there are only two possibilities.

But when you try to fit these with what Tom has, you find that Tom's first possibility forces D... towards only one answer in each case.

When you go on to investigate Harry's possibilities the fact that he says he doesn't know his third forces towards a single solution.

From there it is easy for Sally to sort out everybody's container.

Bob

Hi Bob...thanks for that.

My problem was assuming that the four picked their marbles in the order of occurrence in the story. That assumption is incorrect...each person must use information that has previously been revealed by others.

I ditched my assumption and went on to solve the puzzle. It has only one solution...the one given on the solution page.

This is how I see it (just putting it a little differently from you and omitting the workings).......

For Tom and Dlck to know the colour of their third marble, they must:
- use their labels, and
- know the colour of all Harry's marbles.
We'll also then know Tom's and Dlck's labels.

For Harry to know the colour of his third marble he must know Tom's and Dlck's initial picks.

For Sally to know the colour of her three marbles, she must know:
- Tom's and Dlck's initial picks, and
- the colour of all Harry's marbles.
We'll also then know Sally's label...from which we'll know Harry's label.

To me, the following two consecutive sentences in the puzzle imply that Tom was the first to pick his two marbles and was immediately able to tell the colour of the third:

He then told all of his friends to take 2 marbles out of their container, read the label, and then tell him the color of the third marble.

So Tom took two blue marbles out of his container and looked at the label. He was able to tell the color of the third marble immediately.

The story then lists the others and their picks, and because of my initial incorrect assumption I wasn't able to progress.

So that's where I tripped up, and maybe jr.williams too.