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

I think that the chances are 50%, because if you drew a white marble from bag one, where there was originally a white marble, than the chances are 50-50, because we could say that the white marble Billy drew from Bag #1 was the original white marble (even if it isn't), so then we would be left with a white and red marble, so naturally, the chances are 50% that the marble in bag #2 is white.

Edited on April 17, 2005, 9:21 pm

Posted by Stephen Ticsay on 2005-04-17 21:16:38