C. 1256. A king wants to give a convict another chance to escape prison. The convict is blindfolded, and instructed to select one ball from each of three urns. The guards place the three balls in a fourth urn. Then the blindfolded man draws a ball from this fourth urn. If the ball is white, he is let go free. What is the probability of escaping prison if the number of balls of various colours in the three urns is as follows:

white 
red 
black 
urn 1 
2 
5 
3 
urn 2 
5 
2 
3 
urn 3 
3 
3 
4 

Suggested by J. Czinki, Budapest
(5 points)
Deadline expired on 10 December 2014.