Q: There are N boxes on the table each containing m white and n black balls. Assume you pick randomly one ball from the first box and put it in the second box. Then you pick randomly a ball from the second box and put it in the third box. The process is repeated until you pick randomly ball from the last, the N-th, box.

a) Find probability that the third ball you pick is white if the first picked is black.

b) Find probability that the last ball you pick is white if the first picked ball is black.