Hello mneoxThe 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 isGrandad