Urn A contains n white balls and 2n black balls, while urn B contains n white balls and 3n black balls.
One of these two urns is chosen at random with equal probability and a ball is drawn from the chosen urn.
a) What is the probability that this ball is a white ball?
b) Given that the ball is white , what is the probability that the ball was drawn from urn A?

I'd start by drawing a tree diagram ....

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The greater danger for most of us lies not in setting our aim too high and falling short; but in setting our aim too low and achieving our mark. (Michelangelo Buonarroti)

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