Thread: Marbles in Bags

Bag A contains 3 black marbles and 1 white marble and Bag B contains 1 black marble and 3 white marbles. If two marbles are randomly taken from Bag A and placed in Bag B, then a marble is randomly selected from Bag B, what is the probability that the marble selected from Bag B is black?

I've tried to draw a tree, but I must be doing something wrong cause I can't get the correct answer. Could someone give me some guidance?

Hello mneox

Originally Posted by mneox

Bag A contains 3 black marbles and 1 white marble and Bag B contains 1 black marble and 3 white marbles. If two marbles are randomly taken from Bag A and placed in Bag B, then a marble is randomly selected from Bag B, what is the probability that the marble selected from Bag B is black?

I've tried to draw a tree, but I must be doing something wrong cause I can't get the correct answer. Could someone give me some guidance?

The number of ways of selecting marbles from the four in bag A is .

The number of ways of selecting the white marble and one of the black marbles from bag A .

Therefore the probability that one of the marbles chosen from bag A is white is ; and the probability that both are black is also .

So, when a marble is drawn from bag B, half the time it will be chosen from a total of black and white, and half the time from black and white. So the probability that it is black is

Grandad