A stoplight on the way to class is red 70% of the time. Find the probability mass function for the number of encounters with this stoplight to the first green light. Solution It is green on the first encounter P(1) = .3 It is green on the second encounter P(2) = .7*.3 It is green on the third encounter P(3) = .7*.7*.3 It is green on the fourth encounter P(4) = .7*.7*.7*.3 . . . It is green on the nth encounterP(N) = .7^n *.3.