suppose that 1 in 5 cabs stop when hailed : a. what is the probability that you have to hail exactly 3 cabs until one stops b what is the probability that you have to hail at most N cabs until one stops, where N is a positive integer c how many cabs do we expect to hail until one stops Solution P ( hailing atmost N cars before one stops) = P( N-1 not stopping ) * P ( N stopping) = =(4/5)N-1 *1/5 = 4N-1/5N.