12. Every Sunday, Bob calls Liz to see if she will play tennis with him on that day. If Liz has not played tennis with Bob since i Sundays ago, the probability that she will say yes to him is i/k, k ≥ 2, i = 1, 2, ... , k. Therefore, if, for example, Liz does not play tennis with Bob for k − 1 consecutive Sundays, then she will play with him next Sunday with probability 1. Let Z be the number of weeks it takes Liz to play again with Bob since they last played. Find the probability mass function of Z.