A bag initially contained 4 white marbles and 9 black marbles. A marble was drawn and replaced by two marbles of the opposite colour. Then another marble was drawn and also replaced by two marbles of the opposite colour. A third marble was then drawn.
Find the probability that the first marble drawn was white if:
(a) the second marble drawn was white
(b) the third marble drawn was white&both the second and third marbles drawn were white.