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**Link88** One bag contains 6 white and 4 black balls, and a second bag contains 3 white and 7 black balls. One ball is drawn at random from the first bag and placed unseen in the second bag.

(a) Find the probability that a ball now drawn at random from the second bag will be black.

(b) If the ball drawn from the second bag is black, find the probability that a black ball was transferred from the first bag.

I drew a tree diagram for both of them but the fact that the balls get switched into another bag confuses me : / need some help