The probability of picking a black ball first is [tex]\dfrac{4}{5}[/tex], because there are 4 black balls and 1 white ball which is 5 balls in total. After picking the first ball, 4 balls remain - 3 black and 1 white, so the probability of picking a white ball is [tex]\dfrac{1}{4}[/tex].
So, the probability of picking a black ball first followed by a white ball is [tex] \dfrac{4}{5}\cdot\dfrac{1}{4}=\dfrac{1}{5}[/tex]
