the contents of three urns are :
urn 1: 1 white,3 red,2black balls.
urn 2: 3 white,1 red,1 black balls.
urn 3: 3 white,3 red,3black balls.
two balls are choosen from randomly selected urn.If the bals are 1 white and 1 red ball,what is the probability that they come from urn 2 ?