On a table there are two bags. The first contains one white marble. The second bag contains one red and one white marble. Blindfolded, Billy takes one marble out of the second bag and puts it in the first bag. He then takes off his blindfold and takes a marble out of bag one. It is a white marble.
What is the probability that the other marble in the first bag is also a white marble?

Dutch National Science Quiz 12/24/2004

IF Billy decides a priori to draw a white marble, i.e. he looks, then the probability (for us) that the other marble is white is 1/2.

However, I think the problem implies that Billy drew the marble randomly and it happened to be white, and thus, the probability for everyone is 2/3.

If I remember correctly, it was this kind of twist that caused so many "oops" letters when Savant (sp?) printed the Monty Hall Problem. Did Monty intend on opening a "goat" door?

Posted by owl on 2005-04-19 03:09:55