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 3 cases by The riddler)

Probability doesn't deals only with what could be inside the bag after the marble is transferred to the first bag, but also on the fact that Billy has taken off his blindfold. If Billy wants to pick a white marble the probability he picks it, watching in the bag, is 1, while if Billy is blinded probability is only 3/4 because he can pick unwillingly a red marble. As this probability changes with Billy's condition, it's also subject of change the probability of what rests inside the bag once Billy has picked up.

Edited on April 18, 2005, 4:33 pm

Posted by armando on 2005-04-18 16:00:46