Given that a ball selected in the above process is a red ball, the
probability that it came from the box P is:
P(comes from box p / selected ball is red)
= P(comes from box p ∩ selected ball is red) / p (selected ball is red)
here P(comes from box p ∩ selected ball is red) = (1/3)*(2/5) = 2/15
p (selected ball is red) = (1/3)*(2/5) + (2/3) * (3/4) = 19/30
so required probility is = (2/15) / (19/30) = 4/19