A prisoner is trapped in a cell containing three doors. The first door leads to a tunnel which returns him to his cell after two-day's travel. The second leads to a tunnel which returns him to his cell after three day's travel. The third door leads immediately to freedom.

(a) Assuming that the prisoner will always select doors, 1, 2, and 3 with probabilities 0.5, 0.3, 0.2, what is the expected number of days until he reaches freedom?

(b) Assuming that the prisoner is always equally likely to choose among those doors that he has not used, what is the expected number of days until he reaches freedom? (In this version, for instance, if the prisoner initially tries door 1, then when he returns to the cell, he will now select only from doors 2 and 3.)

(c) For parts

(a) and

(b) find the variance of the number of days until our prisoner reaches freedom.