# Math Help - drawing balls from an urn

1. ## drawing balls from an urn

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?

2. Originally Posted by eyke
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 ....