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 re: Solution by Charlie)

Charlie--

Too many people are afraid of math and will use faulty "common sense" to solve this problem (see the discussion around the very similar -- almost identical, mathematically -- "3 Cards" ), just as they do for The Monty Hall Problem. No matter how many times you show that the odds have to be 1:2 (1/3 probability), they will insist that they are 1:1 (1/2 probability).

Since you are not going to convince them otherwise, you might as well dare them to play these games multiple times, until you have taken them to the cleaners.  :-)

Posted by TomM on 2005-04-17 17:54:24