=+7. Consider an urn with b ≥ 1 black balls and w ≥ 0 white balls. Balls are extracted from the urn without replacement until a black ball is encountered. Show that the number of balls Nbw extracted has mean E(Nbw)=(b+w + 1)/(b+ 1). (Hint: Derive a recurrence relation and boundary conditions for E(Nbw) and solve.)