Assuming that the ball is drawn one after another without replacement, that person could have drawn a white ball first then a black ball or the other way round so you sum up all the possible cases.
(2) (a) You know this.
(b) You know this too.
Among the 3 balls taken out of 5 balls, what is the probability that one of them is a back ball.
OR 3!/2! x 1/5 x 2/4
Since the 3 balls drawn (B,B',B') could be arranged in 3!/2! ways.