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

(In reply to more manually by armando)

If we are looking for the probability of taking a white marble out and THEN taking a second white marble out, the 2/3 figure would work, but the question was what is the probability AFTER a white one has been taken out---you are only dealing with the probability of picking a white one from the remaining TWO marbles--thus 50/50.

Posted by Bruce on 2005-04-18 21:16:48