Answer:
(a) 0.0520
(b) 0.3699
Step-by-step explanation:
Let
be the events of selecting urn 1, urn 2 and urn 3 respectively.
So, 
Let W be the event of drawing a white ball.
Now, probability of drawing 2 white balls from urn 1 is given as:

Probability of drawing 2 white balls from urn 2 is given as:

Probability of drawing 2 white balls from urn 3 is given as:
............. (as all balls are white only)
Now, probability of drawing 2 white balls is the sum of all the above probabilities and is given as:

(a)
Probability of selecting urn 1 given that 2 white balls are drawn is:

Therefore, probability of selecting urn 1 given that 2 white balls are drawn is 0.0250.
(b)
Probability of selecting urn 2 given that 2 white balls are drawn is:

Therefore, probability of selecting urn 2 given that 2 white balls are drawn is 0.3699